Hold your left hand horizontally flat, palm side up. Move the thumb to your first, second, or third finger. Notice how the thumb lies on its left side, which is the position the thumb can/should take in playing. Continue with the demonstration by moving the right wrist in-between the left hand thumb and fingers as though you were taking your pulse with your left thumb. Notice that the thumb is still lying on its left side - good. Now press and notice that the thumb moves from its left side to the power position where it is lying flat with full pad contact - bad. The power position is the position where one reflexively presses too hard. Simply don't go into the power position, which you have to unnaturally force by the way, by keeping the thumb straight, relaxed, and at least somewhat on its left side. This thumb position is recommended by no less than Charles Duncan in part because it encourages light pressure with the thumb.