DartFish is a programme used by sports coaches to video a player and then add comments allowing coaches to use the material to improve performance of their own players. On the time line you will see small white triangles, and as the video gets to these points,click  on that point and the relevant text will appear - give it a go.

Note the different types of finish on the forehand, some over the shouler, and some outside the line of the shoulder, each one giving a different effect on the ball.