This stroke is commonly used against a right handed opponent's cross court forehand loop, in which case the ball will be moving diagonally, but also with a little sideways curve due to the natural tendency of most players to put a little hooking sidespin on the ball when forehand looping cross court.
As the diagram shows, the typical ball flight path is curving away from the player. When the player hits the ball back cross court (the faded bat position), the bat face is much closer to perpendicular to the ball flight path, so it is usually quite easy to make contact.
When the player attempts to play an off-forehand (the solid bat position), the bat face becomes much more parallel to the flight path of the ball. This angling of the bat is what allows the player to hit the ball in a down the line or even left-to-right direction, with extra sidespin to curve the ball even further left-to-right. However, this angling of the bat also drastically reduces the amount of bat face available to make contact with the ball, and the more the bat is angled, the smaller the effective contact area gets.
When you add this to the fact that the direction of the bat's swing is close to parallel to the direction of the ball flight path, you end up with a very narrow window of opportunity to make contact with the ball. This is why we often miss the ball completely when attempting to play this advanced stroke.
