Williams: The EKG is actually is actually the electrical signal in your heart. And then your heart chambers beat, and it pumps blood, but you can tell a lot about heart rhythm by looking at the pulse wave, as the blood flows from the veins. And so that's what the Photoplethysmogram (PPG) on the watch does. It looks at the pulse wave, and we can look at those beat-to-beat intervals, which allows us to look and detect irregular heart rhythms. And we can tune this to look for irregular heart rhythms that are consistent with atrial fibrillation. And that's a big deal. I think you probably know a bit about it, but just to make sure we're all up to speed: AFib (atrial fibrillation) is when your heart chambers beat asynchronously. And the problem with that is, the blood starts to pool, and you know, your blood is designed to clot. That's what keeps you from bleeding to death when you get a cut. And the problem is, if it starts to clot in your heart and then that travels, it can cause problems like stroke. And so AFib is a leading cause of stroke. And stroke is a leading cause of death around the world.

Jeff Williams : It's just a simple study, and people do studies all the time, but we think this is a really, really special time. To date all of the work that we've done with the Apple Watch and the heart rate monitor has been focused on heart rate, and in the last Watch, our last course, we released a discordance feature which allows people to get notifications if the heart detects an eminently high heart rate when it thinks they're at rest. With the (Heart Study) app, the watch will look at heart rhythm.

In Williams' view, the future of medicine involves the patient at the center of the health experience armed with their data. And Apple can play a pivotal role in collecting this information through its massively popular consumer hardware like the iPhone and Apple Watch.

CNBC: And people often don't feel it coming?

Williams: One of the big challenges with AFib is that people with AFib often don't feel the symptoms -- it's asymptomatic. There are pretty clear interventions available, but people don't know they have the problem. So we believe that detecting AFib will put people in a better position to effect their health and hopefully save a lot of lives. That's what this study at the highest level is about. The way this is gonna work, anybody over age 22 in the United States -- there's a few conditions we exclude, people who are already being treated for AFib and other things -- but, in general, anybody can download this app, starting tomorrow. That's a new way to do studies. This is a bit of a breakthrough in that it's a virtual clinical trial. There are probably some other clinical trials that have used apps to some degree, but one broadly available to a large population is one we think will deliver a better perspective. Many trials are done with a limited amount of people and they're based in a certain limited geography, and this gives us a much broader population and the ability for many, many people to participate. And we've taken a very thoughtful approach to this, and we've embraced the medical community. What we didn't want to do was just release an app that said, "we think we've detected AFib" and let the customer sort it out from there.

CNBC: Say more about that participation with the medical community.

Williams: We've taken a broad view of the translational aspects of this to the medical community. So what will happen is, when the watch detects an arrhythmia consistent with AFib, it will notify the user, and the user will then be able to connect with a study physician. Obviously if there's an immediate concern, the study physician could address that. The most likely step that a physician takes after someone has the symptoms of arrhythmia is, they would get an ECG patch. And so as part of this study, the study coordinators will send the people an ECG patch. And the customer will wear that and mail it back in. And then there's a follow-up with the customer. So we're following the customers through and the ultimate goal of this is, is to verify our ability to really empower people with more information about their health with respect to arrhythmias.

CNBC: Understood. And the video aspect with the study physician, is that going to be through a telemedicine-type service?

Williams: It is. We partnered with American Well, and they are going to provide the physician's support, and the customers have a choice of a regular phone call or a video conference. And the video conference is useful, because sometimes you can see things from a physician's standpoint when you're interacting with a patient, so we've built that into the app. The patch is being sent from BioTelemetry and will be pulled back and read and I think it's a really neat opportunity to look at arrhythmia across a broad population. Our primary goal is to detect arrhythmias consistent with AFib with the ultimate goal of reducing strokes and saving lives -- that's the primary goal. What's interesting is there's a real secondary opportunity here to advance heart science. One of the things that became clear to us, when we were working in the study -- we met a lot of key opinion leaders, cardiologists, and there are -- one of the things we discovered is, there are pretty clear guidelines about what to do with someone who has AFib if they are older and have certain conditions. What there is a lack of clarity on is what to do with younger people with AFib. All of the cardiologists we spoke to said there's a real dearth of information, because the prevalence is low and it hasn't been studied adequately. And now we have an opportunity with so many people wearing Apple watches, to contribute in a really different way, and hopefully we can learn a lot about arrhythmia in people who are younger, and contribute to heart science and help people live a better life. And so, that's a really interesting aspect of this that I think a few people will take note of.

CNBC: Absolutely. And I'm wondering, as well, about the regulatory aspect of this, because I'm assuming that you talked to FDA about how to communicate back to a user, you know, "Hey, we think that you might be having an arrhythmia, you should talk to one of our physicians." Can you talk or speak a little bit to that?

Williams: We certainly worked with the FDA. We have Stanford Medicine leading the study, which we think is fantastic. We've engaged with the FDA and we plan to submit the results to the FDA, so we've taken into account all of the appropriate language not to inappropriately alarm people and try to make this a very positive experience for our customers.

Farr: Do you see the Apple Watch as a potential medical device in the future if the study results come back positive and you find that you can use this as a first line of defense health-monitoring tool?

Williams: First off, I can say, I already think that Apple Watch can help people with their health and for their medical activity, but in terms of classifying it as a regulated medical device, I do not view that this study will make the Apple watch hardware a regulated medical device. What the software that the app, or the software, for this particular indication, will be labeled is to be determined. We don't know what that is yet.

CNBC: Got it. And so where did this come from, this desire to want to use the watch to save lives in this way? I've heard on stage these stories of people saying, "the Apple watch saved my life." Was it very organic? Or did you have this vision of it being a tool for this purpose in the beginning?

Williams: You know, it's interesting. It has been a very organic journey. We didn't wake up one day and say, "well we've done the phone, we've done the iPad, why don't we knock out health next." That's not how it happened. We knew when we built Apple Watch that it was the most personal device we've ever made, and we thought we had an opportunity to help people live a better life with their health, but, quite honestly, we've been really surprised with the deluge of letters and emails we've gotten where just the simple heart rate monitor was able to have such an impact on people's' lives. And we've been really inspired and we want to do even more. There's been a lot of speculation about what Apple may or may not be doing in health. But I'll tell you our focus is empowering consumers, empowering individuals to play a more active role in their health. Most of us live in an era, or in a world where you go see your doctor when you're sick, you do what you're told, and that's really it for your view of the medical world. You play a very passive role. You're a passenger in the car. And it's very clear to us that the future is very different. The future is people are going to be more empowered with information about their health and they're going to play a more active role. They'll sometimes drive the car. And a lot of people say, "Well, what does that mean for physicians?" And I don't think it reduces the role of the physician at all. Rather, I think it arms the physician with more information, and it gives them engagement with an engaged consumer -- and that changes behavior. And that's what most physicians would say, "If I could drive behavior change, I could have a real impact on health."

CNBC: I think there's been so much speculation, because Apple is a company that is associated with consumer delight. You think about the healthcare experience and it's just often very terrifying. Is that something that has drawn you into health? And how do you look at the entirety of the health care experience, where there are so many opportunities for Apple to make a difference, whether it's kind of pulling data together into the phone or, you know, tracking health conditions on the watch, and say, "This is what we're going to prioritize"?

Williams: I guess what I would say is there are huge opportunities here. Our focus right now is on empowering the individual. There's lots of places when you pull on the thread where this could lead, but we've got a lot of work to do to empower consumers, get them engaged, and there's just a ton of opportunity beyond that.

CNBC: Is this a personal passion for you? I get that feeling in hearing you talk about it.

Williams: It absolutely is. It's a personal passion for me, it's a personal passion for the members of our health team, and the entire executive team has embraced this. It's not hard to get excited about this. The reality is, if you ever have a loved one that has a serious health condition, everything else you thought was a priority in life falls to the bottom of the list. And we see so much opportunity, and can't think of a better use of Apple resources.

CNBC: And is this a real business for Apple? Is this a business that you envision making real money in, or is it more something that you approach with a philanthropic lens, where you say "our consumers need this and we can do them a service"?

Williams: You know, we haven't spent a ton of time on that, in terms of trying to reconcile those two. I do think there's a real opportunity to empower people, and if in the process they wear more watches and other things, that's good for us. But right now, our goal is to make the best products and services we know how to make, and we assume everything else will sort itself out down the road.

CNBC: You're a hardware guy and super knowledgeable on sensors. Is there any other interesting opportunities that you see in hardware, you know, beyond just the wrist or potentially more on the wrist that we haven't explored yet?

Williams: There's nothing to announce now. What I would say is, when you start measuring the human body, it's really interesting and difficult. Engineers, we often solve difficult problems -- really challenging problems -- but at least the electrons flow the same way every time. And the human body is this constantly changing thing. And so I think there are plenty of challenges but there's still opportunity.

CNBC: I have a personal interest in the medical records space and some of the remaining issues around interoperability of data, like getting health information to one place. So it was exciting for me to go down to Cedars-Sinai hospital recently and talk to some patients who had the health app on the iPhone. They pulled it up and they could see their medical record and their labs and a bunch of other data sources all in one place. Which is a new, relatively new thing. What do you see as the potential here, and how does that lend itself to the mission of empowering consumers to bring this data onto the phone -- making the phone a personal health record?

Williams: You know, we've announced already the ability to import health records. And we believe that every person in the world ought to have their health information. We think the right place for the health information to exist is with the person on their device. And we believe from a privacy standpoint, that where that information gets shared should be -- should be completely up to the individual. There's nothing more personal than your health information. And so we view that as the future. It's really unfortunate that today, uh, the pieces of your medical record and history are spread and sitting in servers of various companies around the world, and it ought to be sitting on the device that you carry every day. And so, that we view as the future.

CNBC: So we might see more from Apple in that area?

Williams: Stay tuned.