This shot involves hitting the ball around the net post (instead of over) and into the other side of the court. This is a legal shot in tennis.
The around the net post shot looks awesome, but the key to hitting the shot is really just getting the right shot to hit it off of. If you watch some of Nadal's best around the net post shots it is clear that you can curve the ball back into the court slightly, but still not enough to pull this shot off from normal court placement. Also it should be noted that in professional singles matches the net posts are moved into the doubles alleys which raises the net slightly, but makes this shot easier to pull off. This trick shot definitely takes some practice, but next time you are pulled really wide, look to see if you have an angle and considering hitting around the net post.