It's been on my wrist for two weeks, and I barely feel it's there. I don't look at it. I don't take it off, except when it needs charging every seven days. It's slim and black, and has no screen.

Jawbone's fitness bands take a totally different approach than recent fitness watches and super-smart smartwatches do: you're not meant to interact with them. The older Jawbone Up and Up24 were like that, and so is the new Up3. Its job is to collect data, sense things and send you to your phone for the rest. This time, Jawbone has added heart rate sensing.

Jawbone's bands never had heart rate sensors before. But the Jawbone Up3 -- which sells for $180 in the US, and will cost £150 and $AU230 when it debuts elsewhere later in 2015 -- uses a unique technology: bioimpedance. That's a big departure from the optical sensor and green LEDs used in competing trackers like the Microsoft Band , Apple Watch and Fitbit's latest Charge HR and Surge bands. And, the Up3 doesn't even currently track your heart rate all the time.

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The Jawbone Up3 band is meant to be a general lifestyle coach, not a hard-core fitness tracker. It's meant to do more as it collects data over days, weeks, even months. You're meant to kick back, live with it, let it understand you.

But even after two weeks, the Up3 just hasn't done enough to justify its price; the heart rate functionality isn't a killer app, nor even -- so far, anyway -- a particularly useful feature. I love Jawbone's Up app and ecosystem, but I'd rather get the heart rate-free Jawbone Up2 band, which delivers all of the Up3's other tracking functionality at almost half the price.

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Design: Understated to a fault

The Jawbone Up and Up24 were CNET's favorites for years because they were low-key, looking like casual sport bracelets, not "wearable tech." The Jawbone Up3 is even smaller -- 45 percent smaller,in fact, than the older Up24 band. It's downright petite.

The rubber band design gets thicker at the top, where an aluminum body houses a touch sensor and three embedded LED status lights. Tap it a few times to see if you're in "awake" activity mode (an orange man icon) or sleep-tracking (a blue moon). The band vibrates when there are notifications to read in your Jawbone phone app, or when a silent alarm goes off to wake you up or tell you to go to sleep...or, to start moving.

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Underneath, metal studs spread across the band, emerging through the rubber. These are the Up3's bioimpedance sensors, which measure heart rate (among other things, eventually). They're meant to contact your skin, so the band needs to be fastened so that it touches your arm all around.

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The Up3 is one-size-fits-all, unlike the older Up bands. But, it's also a lot harder to put on your wrist. A metal clasp on the bottom is adjustable, but it took me minutes to get its position and fit perfect -- not too snug, not too loose. The odd slide-in metal clasp design feels, well, like the clasp on a bra strap. It's fine once it's on, but it's not great to remove and attach again.

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You can wear the Up3 in the shower, or while washing your hands, according to Jawbone, but it's not meant for swimming. It has basically the same water resistance level as previous Up bands.

One thing that's definitely better is how it charges. The Up3 requires an included USB dongle, but it magnetically snaps onto the back of the band. And the dongle's bendable, so you could place it somewhere and have the Up3 sticking onto it, charging.

Sarah Tew/CNET

Up3's strange use (or nonuse) of heart rate

The Up3 measures heart rate using a completely different technology than most current wristbands: instead of a green LED, it uses metal studs to measure bioimpedance. (Older BodyMedia bands used similar technology; Jawbone purchased BodyMedia several years ago.)

Down the road, Jawbone says, these sensors could also be used to measure skin temperature variations, hydration, even stress levels. But, right now, all they're being used for is sleep tracking and resting heart rate. Some of these features are ones that the Basis Peak also claimed to be able to measure, but never seemed to in a meaningful way. But at least the Basis Peak does real-time heart rate tracking for exercise and daily use, and it even has a screen. (It was also bigger, and uglier.)