There are four fundamental safety rules that we consistently teach in our firearms classes: 1) The gun is always loaded until you have verified otherwise; 2) Always keep the muzzle pointed in a safe direction; 3) Keep the finger off the trigger until ready to shoot; 4) Know what is behind your target, and between you and the target. These rules apply to target shooting, and they also equally apply to tactical situations and defensive shooting. Even if you are clearing your home to look for intruders, for example, you need to ALWAYS be cognizant of these four things while looking for the bad guy. And if you are in this situation, I hope your gun IS loaded.
So let's focus on the third safety rule for a moment. Why is it that we teach keeping the finger off the trigger until sights are on target and you are ready to shoot?
Two words: Startle response.
When a person is startled, the natural response is to blink, lean the body forward, bend the arms at the elbows, and clench the fists/fingers. When clenching the fists happens, each clenching finger is able to exert about 25 pounds of force. It only takes about 12 pounds of force to pull the trigger of a revolver in double action mode (hammer down). If your finger is in the trigger and you are startled, you will pull the trigger. If that happens, think about those other safety rules for a moment. Where is the gun pointed when the shot goes of? What is in the line of fire when that happens?
Proper trigger discipline is crucial to ensuring that you will be safe whether at the range or in a defensive situation. Proper trigger discipline is also a matter of building up muscle memory through practice. Beginning with the very first movement of the holster draw, the trigger finger should be indexed along the side of the firearm the minute the hand is placed on the grip of the gun. When at the low ready, the high ready, the retention position, or at the fully extended position, the finger needs to be indexed on the side of the frame. Only when you have a target in sight and you are ready to shoot does the finger move inside the trigger guard. Then simply touch and press the trigger.
Defensive shooting is every bit a matter of safety as it is efficient, tactical movement. Practice, practice, practice. Practice indexing that finger, and practice moving the finger to the trigger. Safely done, dry fire is a key training tool in these methods.
Be safe out there!