Most golfers grow up being told to swing harder, to turn harder, to “use the ground,” and then they go to the range and try to invent speed in the hands. The ball does not care how athletic the move looked. It cares whether force was organized under the feet early enough to travel up the body and into the club before impact.
Ground reaction force, GRF, is simply that push from the floor. When you press down, the ground presses back, and that return is the first real piece of speed in a swing that works. The problem is almost never that a player has no force available. The problem is timing. Press too early and you spin from the top. Press too late and the energy arrives after the ball is already gone. Somewhere between lead arm parallel and shaft parallel, the lead side has to take over, and if it does not, you are left throwing the arms at a ball that will not wait.
You can learn that feel without living in a lab. The Force Pedal sits under the ball of the lead foot so the press has a clear place and a clear moment, soft if you are discovering the timing, firmer when you already own it and want more demand. Feel the ground first, then load it, and club head speed becomes the result of a swing that stopped wasting the force it already had.