Everything said so far is correct, except 0% charge on the watch is not actually 0%, at that point, I'm guessing the watch probably has around 15-20% battery left which is why the watch will work for around two days in power reserve mode, it doesn't run for two days on 0% and power reserve mode uses so little battery because it's basically doing nothing unless you want a quick glimpse of the time, that it probably only uses 5 of that 20% battery that is left. Apple isn't going to let you damage your battery by deep discharging that easily, that would be a lot of warranty replacements.
Also, don't take my numbers above as fact, I don't claim them to be, but you get the idea...