Comment by senkora

These short videos should clear it up:

Backstroke start technique: https://www.youtube.com/watch?v=xwq-IsGNa28

Backstroke flipturn technique: https://www.youtube.com/watch?v=B-WNtRGwSjQ

Underwater dolphin kicks can be done on your front, back, or (I guess) side, so it also works for backstroke. And the way that starts and turns work in backstroke still puts you underwater at the start of each lap.