The inward block drill is a very simple and useful block. This version of the drill pushes the opponents hand inward - while this is good for reflex, in a proper defense situation you should never wait for the opponents hand to come out before deciding which hand to block with - you will chose the block prior (eg left inward block) and use that for whichever hand the opponent strikes with.
Partner punches with their right and you block their hand inward with your left
Partner punches with their left and you block their hand inward with your right.
Partner again punches with their right and you block their hand inward with your left
Continue with the drill for 20 blocks then change so your partner is blocking.