The following is from a digest of a paper on Point Shooting which was written by Walter J. Dorfner, the former lead firearms instructor for the Vemont State Patrol (now deceased). It addresses DA and SA trigger pull. A link to the digest can be found below.

Field Shooting With the Middle Finger

When the middle pad of the middle finger was placed on the trigger, the force needed with double action to cock and fire the weapon, felt much lighter than the measured 12 pounds.

With single action, the 4 pound force that was needed to fire the weapon, felt like simple air resistance.

Also, as the middle pad of the finger was on the trigger, the curling action of the tip of the finger did not affect the fall of the shot.

Another benefit was that the centerline bore was more closely aligned with the web of the hand. That provided for both a natural pointing of the weapon and better control of recoil forces.

New shooters with limited hand strength, had a problem pulling the trigger smoothly with double action when the index finger was used to pull the trigger. That was not true when the middle finger was used to pull the trigger.

Finally, the act of extending the index finger along the side of the gun, helps to extend and "lock up" the wrist which adds strength to the grip, improves recoil control, and minimizes or eliminates the tendency to shoot low and left.

Here is a link to the digest of Walter J. Dorfner's paper on Point Shooting.