I discovered this by accident.

I used to have a Pre and later upgraded to a Pre2, however I kept the Pre's battery as a spare that I carry when I travel.

Returning from a day where I needed the spare battery, I wanted to top it off before swapping the now-dead main battery, so I connected the USB cable to my laptop and let it charge.

When it hit 100%, I opened the back cover, removed the fully-charged spare battery, quickly inserted the dead main battery for charging and lay it back on my desk, expecting to see the phone rebooting.

Instead, it was already on & charging, with the battery at 0%!

When I retraced my steps, I saw that since the phone was being charged the whole time, there was a window of about 3 seconds where the phone stayed on without any battery. As long as another battery was swapped in during that window of time, the phone didn't go into its usual, annoying 5-10 minute reboot.

Live long and prosper