[kraigwy]

The firmer you grip the handgun with your shooting hand, the more the sight alignment will be disturbed by moving the trigger finger.

Take the gun in your shooting hand. Grip it as hard as you can. You'll notice the gun starts to shake. As you release the grip the shaken slows down and eventually stops. At that point, the trigger can be pressed with the less likelihood of disturbing the sights.

Now take the gun, with a moderate grip with the shooting hand, grip it hard with the support hand. You'll notice the trigger finger has freedom of movement without disturbing the sights.