OS X Yosemite’s new user interface has updated icons and translucent windows, but far and away the most interesting feature in the new operating system is "Continuity," a mix of technologies that Apple says will help people move back and forth between their phones, desktops, and laptops.

The first component of Continuity announced by Apple SVP Craig Federighi during Monday's WWDC keynote was Airdrop. Airdrop has long allowed OS X devices to share files over Wi-Fi, but iOS devices have been excluded (even though as of iOS 7, there’s an "Airdrop" button available when sharing media—it unfortunately doesn’t do what you might think it’s supposed to do and let you send files). This odd disconnect has been now remedied—as of Yosemite (and presumably iOS 8), Airdrop can be used to quickly send images and other objects between OS X and iOS devices.

Beyond Airdrop, Continuity has another feature called "Handoff," which appears to leverage both Wi-Fi and Bluetooth so that iOS and OS X Yosemite devices can discover what they’re doing and trade information back and forth. This mixing of location awareness with application-state awareness appears to make possible a number of neat actions, like sending non-iMessages SMS messages from a Mac or using your Mac like a speakerphone with a connected iOS device.

These features were all shown on stage by Federighi during his demo. Federighi pulled up Spotlight and typed in "Dr. Dre," which pulled up Dre’s contact info from OS X’s Contacts app. Federighi then dialed Dre from the Mac with his phone still in his pocket; Dre answered and the call proceeded about as well as a speakerphone call made on a stage in front of thousands of people could be expected. Quality seemed fine, and there were no apparent delays in the handoff part of "Handoff."

The Application awareness component of Continuity (also under the "Handoff" feature banner) is easy to explain: if you’re writing an e-mail or something in an app on your Mac, you can tap a button on your iOS device and the half-written e-mail will appear on that device so you can continue working. Or vice-versa—if you’re busily hammering out a Numbers spreadsheet on your iOS device while commuting home from work on the train, you can walk in the door to your home, click an icon on your Mac, and the Numbers document will appear there for you to keep editing it.

Continuity is one of those concepts that sounds simple to implement, but the devil will be in the details—if the screen switch is fast and seamless (or close to it), it could be a killer feature. However, because it’s something we’ve seen in sci-fi films many times before, imperfections in the implementation could sink it entirely. If the sharing or handoff takes more than a brief moment, the feature might alienate instead of attract users.

It's actually no wonder that this kind of feature has appeared in the past only in isolated desktop/mobile applications rather than in an entire operating system—it's likely a very hard thing to do right. That requirement of a shared application state has stymied most companies from actually implementing it; even with Apple’s Yosemite and iOS 8, the extent to which the feature will be available isn’t yet clear. Will it work only with Apple apps, or will there simply be an API available to add "Continuity" hooks to any app?

Federighi also mentioned that Continuity makes it easier to create "instant hotspots" a feature that appears to work by allowing Macs to know when hotspot-capable iOS devices (i.e., iOS devices with cellular service that allows iOS’s hotspot feature to function) are nearby. Instead of having to pull out the iOS device and manually turn on the hotspot functionality, Continuity appears to allow you to flip it on from the Mac and set the hotspot’s Wi-Fi password, enabling other devices to start joining and sharing.

Unfortunately, further details on Continuity are scarce. As we learn more, we'll be sure to share it with you.

OS X Yosemite will be released in the fall for no cost; developers will get access to the beta today. Additionally, non-developers can sign up now for Apple’s new public beta program and download beta builds of the OS themselves.