In dressage, you ask your horse to bring his hindquarters under his body, lift his back and withers and take a contact. But when you grip with your knee, causing your leg to swing back and your upper body to tip forward, you block the energy you’re developing from behind. So it’s like driving with the parking brake on. With your lower leg, you’re telling him that you want him to go forward, while your thigh is restricting the energy when it grips to support your upper body’s forward lean. At some point, he stops listening to your subtle cues.
In addition, many riders grip with their legs incorrectly just to try to hold themselves still on their horses for flatwork. A truly “still” rider cannot maintain the proper motion…