To make a shot go higher, it's tempting to lift your upper body and try to help the ball in the air. It's also a reason you don't hit iron shots solidly.
To hit low or high shots with your irons, your posture should stay exactly the same. The only thing that changes is how your arms and hands release through impact.
A very easy way to see this is to throw a ball down the range underhanded with your right hand from your golf setup. Throw the ball down directly in front of you, and you've got the feel for a low shot. Throw the ball into the air and down the range, and you've got the feel for a high shot.
