The percussion effect during this section is played slightly differently to how we achieved the effect before, as in with the thumb. What is more effective here is to clench the hand into a fist and let it fall onto the strings so that they hit against the fretwire. Keep the hand nice and relaxed though as it will achieve the best results. Utilising this technique is a little more efficient than using the thumb to create the effect as there it helps to prepare the hand to play the chords directly after the percussion. Go ahead and try it with your thumb though so you can see and experience the difference between the two.