i actually did pretty well with the kingside moves, but the queenside stuff with a4, b3, dxc5, i don't really understand.

ok, 12.a4 maybe stops ...Bb5, exchanging off the light squared bishops.

i guess 16.dxc5 is to clear the a1-h8 diagonal, capa must have seen that the c3 pawn would be moving and Bb2 would be good.

ok, actually it makes sense now. i guessed Ng5 a few times rather early in the game, but before beginning kingside operations with Ng5, Capa "develops" his a1 rook by exchanging it, and plays dxc5, and prepares Bb2. if black doesn't play 17...b4, Capa will just play Bb2 and c4.

by the time Capa plays 20.Ng5, he has settled the queenside and has developed or exchanged every piece. which is much better than playing it on move 12, which is what i would have done.

this reminds me of the section of that vukovic book "the art of attack in chess" when he talks about preconditions for an attack, and how Capa would often, upon gaining a potential attacking position on one side of the board, settle the position on the other side of the board to prevent any possible counterplay before proceeding with the attack.