"If the serve, after bouncing on the receiver's court, bounces backwards, it shall be a let serve."
This would make it very difficult (not necessarily impossible) to serve so shorter players can't reach it. I've seen players use these serves against younger players and it's sickening. I once coached a very short 2200-rated kid against a 1700 player who did this on every serve, and the kid lost, deuce in the fifth, with both winning nearly every point on their serve. (The other guy got the match and rating points, but everyone booed him.) As an exhibition trick, I use to practice these serves so that the ball would bounce backward to my side on the first bounce, and once did 14 in a row.
-Larry Hodges