As mentioned in the previous diagram's explanation, if Player B misjudges the placement or spin of Player A's serve, there is a chance that he may believe the serve is going to go to his backhand, and start to prepare to hit a backhand return.
If the serve then crosses over the playing elbow of Player B, he must decide quickly whether to change his stroke and play a forehand instead, or to continue with his backhand stroke.

If Player B changes his stroke to play a forehand, he will find it difficult to make a good preparation and a powerful stroke, since he will have to abort his preparation for his backhand, and try to start again on his forehand, all while the ball is still travelling towards him.

On the other hand, if he continues to play his backhand stroke, he will need to move over to his forehand side to reach the ball, as shown in the diagram. This will pull him out of position and open up a gap on his backhand side.

If Player B moves to cover the gap on his backhand, Player A can place his attack out wide to Player B's forehand, which will be likely to catch Player B moving in the wrong direction and unable to reach the ball. If Player B holds his position instead, Player A can aim at the wide gap on Player B's backhand side, and Player B will again find it difficult to defend the attack. Finally, if Player B only takes a half-step back to his backhand, Player A has slightly narrower gaps on both the wide forehand and wide backhand, either of which should be a winner if Player A can hit his mark, and still difficult for Player B to answer with a strong shot if Player A is anywhere close to his targets.