Some people think that you need to sort of "pop up" when you make contact with the ball to get added power.
I do not.
When you bend your knees and shift your weight through the shot, notice that as your knees sort of naturally "ratchet over" you do not stay perfectly level. A ground stroke is a co-ordinated mess of balance, motion and timing. It is no time to add an exta "popping up" motion.
At any rate, you want the majority of your mass moving foreward in the direction you are hitting the ball. Your follow through, low to high, will give you the topspin necessary to keep the ball in play.