Their hammers are at rest against the frame with a gap between the face of the hammer and that portion of it which would impact the firing-pin. When the trigger is pulled, the transfer bar rises up to interpose between the hammer and pin, filling the gap. Thus, the hammer hits the bar which "transfers" the hit to the pin.
S&W's make use of of a hammer-block, and a rebounding hammer. With your S&W TRIPLE-CHECKED AND VERIFIED UNLOADED, you can do this little exercise WITH YOUR REVOLVER POINTED IN A VERIFIED SAFE DIRECTION. :eek:
Pull and hold the trigger back. See how far the hammer sinks down between the sides of the frame. Now, slowly release the trigger and watch the hammer rebound, that is, to move back away from the firing-pin. It's at this point that a hammer-block moves up in front of the hammer to fill the gap between it and the frame. This blocks the hammer from contacting the firing-pin in the event of a blow to the hammer, or in the near impossible event the rebound engagement surfaces should wear down to a point of unsafe condition.