What I want to say is, you won't be behind a tree with my method... BUT, if you are:
I don't teach a special shot like a "knockdown" per say, for a couple reasons. Golf is hard enough while you're playing and harder when you're in trouble. Trying to use a shot you don't practice much or use very often, is a "low percentage" shot to try. I'd prefer, to choose the iron that is designed at the trajectory I need and just get it back into play. This may be a 4 iron and just choke it down and punch it out safely. Do not try to make it go farther than is needed, trying to make up for being behind the tree. Take your medicine, get out of trouble and don't add insult to injury. Now that your back in play, use this next shot to help make up for being in trouble. If you're having trouble keeping it low, you may be flipping a bit at impact adding loft. Practice at the range letting your body rotation, bring the wrists through impact, a bit exaggerated with the front wrist bowed out (towards target), back wrist pushing in helping hold the shape, and the face square (or you'll shank it). This will keep the ball lower. Kind of a punch shot. The body must be turning continuously on the downswing.
Move Less ... Get Good! DUPLESSISGOLF