I just got back from two back-to-back international trips, which meant some really long airline flights. What I didn't realize was that with watchOS 3 Apple changed how you put your Apple Watch into Power Reserve mode. For those that don't know, Power Reserve mode puts your watch into a power saving mode (no bluetooth or background processes are running). It displays the current time in a special "Power Reserve" watch face and it only does this when you press the Digital Crown. This is a nice feature because most of the time you aren't getting notifications on an airplane anyway (unless you purchase wifi for your phone) and if you are on a really long flight it is nice to save on power.
So for my first trip when I got on my flight I tried to access Power Reserve mode like I always had before (which was to long press the side button and then swipe the slider to activate Power Reserve). But this is no longer how you access Power Reserve mode with watchOS 3. I accidentally found how you do this though on my 2nd overseas trip. In watchOS 3 if you want to activate Power Reserve mode you swipe up from the bottom of the screen to bring up Control Center:
...and then press the battery percentage indicator. This takes you to a new screen that displays your battery remaining percentage and at the bottom of that screen there is a slider you can use to put your watch into Power Reserve mode:
It can be confusing when Apple makes major updates to the operating system like they did with watchOS 3. Features like Power Reserve can sometime get shuffled around into a new section of the user interface. This happened with watchOS 3 because Apple changed how the Digital Crown and the side button on the watch function. So if you were wondering where Power Reserve went, wonder no more.