Gruber, wer sonst. [daringfireball.net]

"This feature, which Apple calls “Activate on Wrist Raise” works pretty damn well. It’s not perfect, alas, but it’s far more accurate than I feared it would be. The way it seems to work is that if the watch thinks you’re looking at the face, it turns the screen on for about 6 seconds, then turns it off again — even if you’re still holding your wrist in the looking-at-it position. If you turn the display on by tapping the screen or pressing the side button or digital crown, it stays on for about 17 seconds before turning off. I presume the difference is because it’s far more likely that you’ll trigger a false positive for a wrist raise than that you’ll accidentally tap the screen or press one of the buttons. So, the display only stays on for 6 seconds for a wrist raise to avoid wasting battery life for false positives.

In Settings: General: Activate on Wrist Raise, you can turn this feature off. (It’s on by default, and I think the overwhelming majority of users will keep it on.) When it’s on, you can also specify where you go when the screen activates: Clock Face (the default) or Last Used App. Let’s say you’re using the Messages app. When you’re done, you just lower your wrist, and the display will soon go to sleep. By default, the next time the display wakes up you’ll be back at the clock face. (Unless you wake it back up within a few seconds of it going to sleep — in that case it does the right thing and keeps you where you were, regardless of your settings.) If you change this to “Last Used App”, you’ll instead be right back where you were when the display went to sleep. “Last Used App” makes Apple Watch work more like an iOS device. But it’s not an iOS device, and I think Apple’s default here is correct."