If they are punching and you reach out toward their punching arm to block or to attack their arm, you are exposing yourself. You are not only acting after them (so you are behind the timing), but are committing yourself to block or attack their arm or whatever you want to call it (and you can't "interrupt it like many believe since as his punch is so fast, your response needs to be even faster since you are reaching). Good boxers/strikers will eat you for lunch if you do this. Good wrestlers will shoot in as soon as they see your hand reach out to block.