How to squeeze the tennis ball, a golf comparison
Squeezing the ball in the racket defines how long the ball will be in contact with the strings and also the resulting power produced by the strength vector generated. An easy way to see the effect of squeezing the ball is to watch a golf ball at impact with different clubs. The low number clubs have a very small face angle, they are almost perpendicular to the ground. The higher the club number, the bigger the angle of the club face. On the image the bigger angle belongs to a sand wedge. The shape of the ball is almost preserved at impact. With the driver, almost perpendicular to the ground, the ball is absolutely out of shape. In tennis, to have the ball for longer in contact with the strings is important because it gives the player a better control and a stronger, more potent ground stroke. Professionals try to have the racket perpendicular to the direction they want the ball as much as they can. It is very effective. To get the most potent stroke one should also try to push the racket on the same direction he is trying to hit the ball. The follow through on the same direction.
At contact, the angle of the racket depends on the stance, on the grip, on the racket trajectory and on how the player directs his body on the stroke (angular vector or linear vector). With a western grip, on open stance, usually the player rotates his body to hit a forehand and drives the racket up to make contact with the ball. That makes it impossible to squeeze the ball. The player might get a good amount of top spin, but no power at all. Also the chance to hit the frame of the racket instead of the strings increases. Using an eastern grip, either open stance or square stance, the player will push his body forward and drive the racket in the same direction. He will have a stronger stroke and will frequently hit the ball at the sweet spot of the racket. That means he will have better control.