I believe this is the fail-safe reboot code, but if you press and hold the middle button for about 15 seconds the watch reboots.
I found this out the hard way. My game, watch rogue, turned on button repeat so you could go for long walks by just holding down a button. This means, however, that if you go for too long a walk, the watch will unceremoniously reboot making it look like my game died.
1) Does this mean we shouldn't use key repeat on SELECT? Because if you do, it is only a matter of time until someone hits this in your app.
2) Can pebble change the reboot sequence to something a bit more complicated? Like holding more than one button down for 15 seconds? My vote would be something involving the BACK button as that is already pseudo reserved: BACK + SELECT for 15 seconds seems unlikely to be accidental.