Now before I tell you what should be happening at impact I want to clear up something very important about the impact position. And that is this:
 

Important Note: When you arrive at the impact position you should NOT be in the same position as you were in at address.

A lot of golf teachers teach this but if you look at all the top ball strikers at impact this simply does not happen. For example....

David Toms Address Position                    David Toms Impact Position

So the first thing we're going to have a look at in regards to impact is the hands, and more specifically your left hand. In the 1956 April edition of Golf Digest Ben Hogan wrote this:

And a few minutes ago I said that you should not be in a position at impact that is similar to what you were in at address.

Remember?

Well, one part of your body that should definitely not be anywhere close to the position it was at address is your hands. Because at impact your left hand should be convex slightly (bent slightly towards the target) and your hands should be quite a bit in front of where they were at address (look at those David Toms pictures for confirmation of this). This should happen naturally when you swing, because your hips move well in front of where they were at address due to the great lower body movement during the downswing.

But sadly a lot of golfers interfere with this natural action, and instead they try to scoop the ball to help it get up in the air. The result of this is a left hand at impact that is concave. This very weak position with the left hand results in golf shots that lack accuracy, power and penetration.

Important Note: If most of your irons go much the same distances, e.g. a 3 iron going much the same distance as a 6 iron etc. the reason will be because at impact your left hand is concave.

Now remember when I said that right hand is the power hand. Well it is, but the left hand is the lead hand and it must control the power applied by your right hand.

You can't let your right hand lead the downswing. It must power it but the left hand should always lead it. And when become a student of mine and learn to hit the ball with your left hand in the position of being convex at impact you'll hit shots with a lot more power and a much better flight trajectory that will add distance to your shots. Not to mention, much greater accuracy too!

Now at impact your right heel should be up off the ground. This is a result of most of your weight (about 90%) being transferred to your left side during the transition and downswing.

Also, both of your knees should drive towards the target to help the lower body get through the shot with the most power possible. Because of all of this your hips should be about 6 inches in front of where they were at setup, and open by about 45 degrees to the target at impact.

Now listen up to what I have to say next because this is very important.

Even though your hips should move well towards the target and they should be very open, your head should be in pretty much the same position it was at address. You should not allow your head to move towards the target until well after impact.

By doing this it means your lower body is well in front of your upper body which is just the way it should be. And the reason you want your lower body to make such an aggressive move towards the target and then to open up is to clear the lower body out of the way -- especially the hips, to allow the shoulders and arms a lot of room to swing down forcefully to impact and into the follow-through.

OK, we've covered a lot of detail so here is a pictorial summary of what should be happening at impact from head to toe:

                                    Right heel off the ground

And finally for this section, here is a summary of the six main things that are critically important in your transition, downswing and at impact.

1. Weight Transfer/Hip Slide/Rotation: At impact about 90% of your weight should be on your left side and you hips should be open by about 45 degrees to the target. Also, at impact your hips should be about 6 inches in front of where they were at impact.

2. Shoulders: At impact your shoulders should be slightly open.

3. Head: At impact your head should be in a position very close to the one it was in during setup.

4. Late Release: When your left arm is parallel with the ground the shaft and your left arm should form about a 45 degree angle (or less).

5. Hands: At impact your hands should be well in front of where they were at impact and your left hand must be flat and your right wrist must be flexed.

6. Shaft Plane: At the halfway point in your downswing your shaft should be pointing at the ball or to the right of it.

So that's the six most important aspects of the transition, downswing and impact. Get them right and everything else will almost certainly fall into place.