Surprised why youg uys don't know this, but BSB (boost select boost), is a way to gain some extra milliseconds on a speed lap by resetting the lap twice before the start of a new lap. It's always used with air time, easy example is ubermall

Ubermall, you would boost on the climb on your way to the start/finnish line, as soon as you boost, you press and hold the lap reset button to get back another turbo, while still in mid air because of your previous boost, you use the newly acquired boost from the lap reset right before the start/finnish line, gaining you the same boost as usual, but now you can also do a barrel roll on top of the normal boost
It's easier to understand with a video, so here you go

Example; https://www.youtube.com/watch?v=WVCY5z38nkw

2 boosts are used before the lap starts

Cipher