That's where it gets subtle. On a Mac, the icon for those apps appears on the left side of the Dock (or will, once OS X Yosemite ships next month). But on an iPhone or iPad, there's no such obvious if unobtrusive notification while you're working. As the figure below shows, Handoff announces apps you can take a handoff from in two places:
Through a tiny, easily overlooked icon at the bottom of the lock screen. Swipe up to open it.
In the App Switcher, if you swipe to the left. (Double-press the Home button to open the App Switcher.) Tap the app to open it.
When you open that app via Handoff, any data you're working on is carried over from the other device, such as an event you're adding in Calendar or a message you're composing in Mail. The feature works, if you know to look for it. I think Apple should provide the option for Handoff alerts in the Notification Center to give people more awareness when an app handoff is available.
The extras Apple has for only the iPhone. iOS 8 brings to the iPhone 4s and later several extras beyond core iOS 8 that I really appreciate.
One is the new Health app, which collects health data from apps and devices that you connect to your iPhone via Bluetooth, such as fitness monitors. But like the iPhone 5s, the iPhone 6 itself is a fitness sensor, so you can immediately capture information like how many steps you've taken and how far you've walked. Most fitness monitoring gear sits unused after a few months, but your iPhone is almost always with you. For those of us who should be more active, the iPhone could be a more realistic way to do so.
I also love that the Health app stores your critical medical info and can make it available to anyone via the lock screen's Emergency button. For example, if you're incapacitated, a caregiver can get vital information immediately. What a great idea!
Another cool feature is the new iPhone Cellular Calls feature (enabled in the Settings app's FaceTime panel). If your phone rings, any iOS 8 or OS X Yosemite device you have within Bluetooth range will also ring, allowing you to take the call from it. On an iPad, iPod Touch, or Mac, the phone call is sent via FaceTime over Wi-Fi. There's a lag of a second or so when using this feature, as the voice traffic gets sent between the iPhone and your other device, but it's a great way to pick up a call when your phone is not at hand.
The iPhone Cellular Calls feature is not part of Apple's new Handoff technology, so it works with more old Macs (as long as they run OS X Yosemite) and old iPads and iPod Touches (as long as they run iOS 8) than Handoff does. (Handoff is restricted to Lightning-equipped iOS 8 devices and 2012-and-later Mac models running OS X Yosemite.)
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