Howdy
Picture is worth a thousand words department:
This is the hammer from a S&W Model 17. All S&W hammers are basically the same, this one just happens to be to a rimfire revolver. Otherwise it is just like the hammer in a Model 14. The arrow is pointing to a part called the Double Action Sear. This part pivots on the pin near the front of the hammer. There us a spring inside the hammer that presses the Double Action sear forward, but allows it to retract into the hammer as needed.
This is the trigger from the same Model 17. Identical to most S&W triggers. The arrow is pointing to a portion of the trigger called the Double Action Sear Strut.
In this photo, the parts are in position and the lockwork of the revolver is exposed. The hammer is down in the normal 'at rest' position. The upper arrow is pointing to the Double Action Sear in its normal extended position. The lower arrow is pointing to the Double Action Sear Strut on the trigger. A lot of the trigger is obscured by the hand, that is why I posted the photo of the trigger with the arrow separately. When the trigger is pulled double action, the Double Action Sear Strut will rotate up. It will engage the bottom of the Double Action Sear on the hammer and rock the hammer back. As the trigger continues to be pulled, at some point the Double Action Sear Strut will slip past the Double Action Sear and allow the hammer to fall. When the trigger is released the Double Action Sear Strut will shove the Double Action Sear into the hammer for a moment as the strut passes by. Then the internal spring will pop the Double Action Sear forward again, ready for the next shot.
A lot of target shooters in those days were taking part in what was known as Bullseye matches. The Model 14 was very popular in these matches, and the shooters were always shooting single action, never double action. So for those shooters, there was no need for a double action trigger. All S&W did with the Single Action Only Model 14s was omit the Double Action Sear and its spring from the hammer assembly. The revolver would function normally in single action mode, but if the trigger was pulled double action, there was no Double Action Sear on the hammer to be engaged by the Double Action Sear Strut on the trigger. So the hand would rotate the cylinder normally, but the hammer would not move.
Actually, if you watch the video carefully you will notice that when the trigger is released, the hammer rocks back slightly. The same thing happens with a normal S&W revolver when the trigger is released. The Rebound Slide (the part behind the trigger) is doing its normal job of shoving the trigger forward. As it does so, the bump on top of the Rebound Slide wedges the bump on the bottom of the hammer up, pulling the hammer back slightly from the frame, as you see it in this last photo. Notice the Hammer Block (the slanted piece) has been shoved forward and up by the pin on the Rebound Slide, but the hammer is not actually contacting the Hammer Block. All completely normal, the Hammer Block is actually a redundant safety feature, there only in case something hits the hammer hard enough to break the bumps on the bottom of the hammer and the top of the Rebound Slide. Then the Hammer Block will prevent the hammer from going all the way forward and discharging a cartridge. A standard feature of all S&W revolvers since 1944.
Yes, S&W did offer kits to turn a Standard Model 14 into a Single Action Only one. The kit had a Single Action Only hammer without the Double Action Sear. I don't know if a new trigger was supplied in the kit or not.