A proper grip helps better manage recoil and muzzle rise, and helps a shooter with accuracy, speed, and retention. The hands should be as high on the gun as possible. The handgun should fit in the hand to be straight like an extension of the shooter’s arm. The support hand’s fingers should wrapped around the front of the strong hand, so that all four fingers overlay the three lower fingers of the strong hand. While the strong hand squeezes the pistol with front-to-rear pressure, the support hand clamps pressure from the sides. This gives 360-degree pressure on the pistol providing a stable grip.
Gripping a semiautomatic pistol with thumbs behind the slide is extremely dangerous and can result in serious injury. The movement of the slide across the hand can have devastating and permanent effects.
SOURCE ARTICLE: https://www.agirlandagun.org/how-to-grip-handgun/