The bottom fist strike is started by pulling the arm back and up. Most of the time you are going to start around the ear area. As you launch forward, you want your hips and body to move with the strike.
As you move forward, you want the butt of the stick to drive through in a downward motion. Picture yourself as if you are driving the stick from the point of contact all the way to the ground.
The arm, hand and hips move together to cut all the way through the target area. As you are making this movement, think of the stick crossing your body. Keep the movements close in to the center of your body allowing you to quickly follow up with more strikes.