The key to a thick rough flop shot.
When you’re playing a flop shot from thick rough it’s almost like a bunker shot. It will be hard to get spin on the ball so you have to use height to get the ball falling onto the green very steeply so it stops quickly. You need loft and clubhead speed through impact to get the necessary height. Any kind of deceleration through impact will lead to a chunk or you’ll dump the ball in the bunker in front of you. It’s much easier for amateurs to stop the ball with height than spin.