Premoves

How do premoves work? I premoved a rook to capture a bishop and I thought that if the rook couldn't capture the bishop after their next move the premove would be cancelled, the bishop moved and I ended up blundering the rook lol.

I feel like I've seen it be the way I describe it before but maybe I'm just crazy and tilted

Pre-moves are based on the square; it is not based on pieces.  As long as it is a legal move following your opponent's turn, your piece will move there.

The premove only works if it's possible. so say you premove recapturing in anticipation of them taking a piece and they don't take the piece, then it cancels. In your scenario you premoved the rook move, and if the bishop moves, the move is still possible so it goes through and becomes a blunder.