ORIGINAL: davepjr71
The rest cord on all of the QAD's only purpose is to bring the arm to parallel with the string. Length has no effect what so ever on how fast it drops. The force the arrow exherts on the arm sets the arm in motion and the spring pulls the arm down. If the cord is not under tension at full draw then the rest is not set-up properly and you will get contact problems.
The instructions clearly state that if the arm is not dropping fast enough, then you need to lengthen the cord. Doing this makes the arm react sooner so it can get out of the way of the fletchings.