Hi, Lee.
To understand, take the sideplate off an S&W revolver (you should have relieved mainspring (hammer spring) tension before doing so) and take out the mainspring.
Then operate the double action slowly (watch that the edge of the hammer doesn't hit the frame and dent that little point). As the trigger cams the hammer back using the hammer strut (S&W calls it the sear), there is a point where a cam on the trigger engages a surface on the hammer, the bottom of the part the single action notch is in. As the trigger moves back further, that surface takes the tension and the strut is free, that is it no longer bears on the top of the trigger. It isn't too easy to explain but I think you will see what I mean if you watch the action.
In a Colt or most other revolvers, as the trigger pushes the hammer back, the strut and the top of the trigger start to become parallel, plus the spring tension increases. The combination causes the trigger pull to become progressively harder, a result known as "stacking." By using that camming action, S&W changes the trigger-hammer leverage at a critical point and the trigger pull remains at a constant tension throughout, enabling more accurate double action firing.
(FWIW, that camming action is what is called the S&W "short action", as opposed to the older long action which did keep stacking to a minimum through the shape of the trigger and strut.)
Jim