How double-tap responds varies based on what you’re doing in the watch. For example, if doing nothing and just on the watch face, a double tap will open up the list of widgets (called the Smart Stack on WatchOS 10). Whereas if a call is coming in, it’ll answer the call. And inversely, if on a call, it’ll hang up the call. Alarm ringing? It’ll snooze it. The double tap action is tied specifically to the primary button in any given app/scenario. Though not all apps do anything with it yet. For example, in sport recording modes, nothing happens.
In testing it out in the hands-on area, it works reasonably well. The first time I put different watches on my wrist I tended to need to adjust/tighten the straps a bit before it triggered, but once I found that ‘happy point’, it triggered flawlessly each time.
Note this is different than the accessibility-focused gesture-based feature that Apple has had in their watch. To begin, on the accessibility feature, that’s more limited in how it gets triggered, and is tied to specific components of the UI. Whereas double tap is running 24×7 to catch those double tap gestures. Further, 3rd party apps can leverage the double tap trigger points as well, and react accordingly. Again, there is overlap here, but it’s not quite as much as some think. Apple says the new S9 chipset is required to run this algorithm 24×7 to capture those actions.