Side-spin on groundstrokes is nothing new, but usually when it is employed it is from a specific side. For instance, a left hander that puts sidespin on a forehand will usually pull the stroke from 9-3 on a clock face to cause the ball to move leftward. To go in the opposite direction is almost against biomechanical laws but somehow Rafa has found a way to do it. I first saw him do it against an opponent who stood there at the net as he watched a ball sail down the alley apparently out of bounds only to watch it curve inward which never happens in normal earthbound tennis. Among the gods of tennis this may be a normal event but to see it brought down from on high was astounding to the player on the receiving end, to the announcers, and to me. Rafa just introduced a new shot.
Look for his side-spin pass among this group.
https://youtu.be/jDvR6tgUMLg
Also, go to minute 7:22 to see what looks to be the side-spin curve forehand.
All of his passes are great but he has added a new level of deception to making the pass.