Operating cords

In this pic, the red cord, attached between Percy's shoulder blades, comes under tension as Percy slides to the right. This causes him to straighten up, reaching his greatest height directly over the start of the tracks.
At this point, the yellow cord comes under tension which causes a strip inside Percy's body to be pulled down which operates the linkage to his arms.
Consequently, Percy's 'elbows' move out and the ball is dropped on to the track.
Percy returns to his receiving/crouching position when he slides back to the left.