Handgun Shooting Analysis chart, courtesy US Army Marksmanship Unit. Public domain image.
The above chart is designed for a right handed shooter. For a left handed shooter, the chart is mirrored vertically (e.g.) "Thumbing" is on the left, "Finger not on trigger correctly" is on the right etc.
The chart is pretty self-explanatory. For instance, if a right handed shooter is shooting too much to the left of the target, he or she is not placing the finger on the trigger correctly. However, some of these terms may need some explanation, so we will study those below.
Thumbing: When the shooter is squeezing the trigger, he or she pushes the right thumb and/or left thumb against the side of the frame, causing the front of the handgun to aim to the right (for a right-handed shooter. A left handed shooter will push the front of the handgun to the left). This causes the shots to end up to the right of the target.
Tightening grip when pulling the trigger: The shooter is tightening their hands along the grip, as the trigger is being squeezed. This causes the front of the gun to dip low and to the right.
Breaking wrist: This is caused because the shooter anticipates the recoil of the gun and does not lock the strong wrist properly. If the shooter tries to mimic the recoil, he or she breaks the wrists upward and shoots above the target. If the shooter tries to counteract the recoil, he or she breaks the wrists downward and the gun fires below the target.
Jerking: This happens when the shooter tries to fire the trigger as soon as the moving sights cross the target, adding excessive pressure to the trigger. This causes shots to end up low and to the left of the target.
Finger not on trigger correctly: This is caused when there is too little finger on the trigger. The trigger finger should cause the trigger to pull straight backwards. However, if there is too little finger area on the trigger, it will pull the trigger sideways and cause the shot to end up to the left of the target.
Pushing: This happens when the shooter jerks the trigger finger forward, just as the gun fires. The solution is to practice a proper follow-through and hold the gun steady during and after the trigger is pulled.
Heeling: This is caused by the shooter putting too much pressure with the heel of the hand, while the weapon is being fired.