I've often seen (on lichess, I play only bullet, at ratings 1800ish) black play the f8 bishop to c5, and then if white challenges it by playing their c1 bishop to e3, black retreats their bishop from c5 to b6, and then if white captures, black recaptures with the a7 pawn.
rnbqkbnr/pppp1ppp/8/8/8/8/PPP1PPPP/RNBQKBNR b KQkq - 0 1
1. Bc5 Be3 2. Bb6 Bxb6 3. axb6
Why is this a good move? I've see it CONSTANTLY in a variety of openings, and I myself never play this way.
It seems a bad move for black to me because
a) you lose a tempo.
b) you double your pawns.
c) yes, you do get an open file for your rook, but ... so what, quite frankly? It's a bullet game. You are probably not going to have the time to think of some ingenious rook lift in the middle of the game. It just seems too risky to utilize that a-flank with the rook in a bullet game, at least for the players at my level. Indeed, I can confirm that I can't recall a single game where black utilized the strategy above and actually got something worthwhile out of it. The rook never made an impact on the a-file in a single bullet game that I've played where black played above strategy.
So why do players do this?