The idea is to hit the ball before the club hits the ground.

With irons, the club should be traveling downward as it strikes the ball; hence the saying "hit down on the ball to make it go up." Of course, there are various degrees of hitting down: Some players take a big divot after hitting the ball, and others take little or no divot. But all good players have the face of the club travelling downward as the club strikes the ball. I'm sure that some of our more technically knowledgeable members can elucidate the effects that hitting down on the ball have on spin rate, so I'll leave that to them.

Hope that helps a bit.