Here is how and why you would shoot one-handed with a handgun. You hold the handgun the same way you would with a two-handed grip but then you take away your support hand and you put it behind you, or you put it in a ball up on your chest. You want that support hand out of the way. You might use the one-handed grip if you are keeping a child or a non-shooting adult behind you for safety purposes, or if you are injured in your support hand. You want to cant your gun slightly toward the midline. This provides a bit more stability in your forearm and it aligns the sights better with your eye. The other modification is that you want to grip the pistol even tighter than you normally do to add to additional stability. Other than these changes, your stance, arm position, lean and sight alignment are all the same. The concepts are all the same whether you have to fire the gun with your shooting hand or your support hand.