When a player intends to hit in the center of the strings and misses above the center, the torque makes the racquet tend to open, and when you hit below, tend to close.
Opening would make the ball fly higher and also have less topspin.
With today’s powerful racquets that would be to risk hitting long.
Most weekend players compensate by tightening the grip to help prevent the racquet’s turn.
Professional players don’t like that solution because they play and practice practically daily and gripping tightly ads to the arm’s stress.
They rather have ease, a looser grip, and thereby more feel.
Hitting a forehand drive, for example, attacking the ball from below for some topspin and making contact with the racquet about horizontal, below would mean towards the bottom of the strings, and I mean quite markedly.
Some players hit below and a bit towards the tip of the racquet.
Andre Agassi, in his best days, was a clear example of that.
Today’s top pros do it more consistently than past ones.
Of course, there is a high dependency on timing, but the top pros, in their best moments and best days excel at that.
On volleys or slice strokes, the better tendency would be to hit above the center, but perhaps not quite as much as the grip is naturally firmer and the stroke slower on these shots.