Well, if you're super-fast, you can also hit someone mid-bounce, since they're in the air, they have contact with nothing. But, if the defender is on the back foot, then then his weight is shifted back. If you attack, he's in the ideal position to jam a front leg sidekick or a reverse punch, and you plow right into it (ow! my ribs). If your opponent is moving forward, then they have their weight forward, then they have to "put it in reverse" (shift their weight back) first, to retreat. This is all predicated on the assumption that you're sparring for points, however. In an SD or full-contact situation, you might take a hit in order to close the gap. In a point match, you meet the foot, and lose the point.