A serve hit with heavy topspin that causes the ball to bounce high and jump toward the returner's body or backhand side. The go-to second serve for most pros because the topspin gives you a bigger margin of error over the net while still being aggressive. A great kick serve bounces up above shoulder height and is genuinely uncomfortable to return. Nadal's kick serve to the backhand is a nightmare.
"Kick serve bounces up to head height on the ad side. The returner is jammed trying to get the backhand return back."