Actually, although the hammer block is one of the important passive safety devices built into the gun, the hammer block is not at work in this particular scenario (push off). The primary passive "safety" in the S&W revolver is the relationship between the rebound seat on the bottom of the hammer, and the hammer seat on top of the rebound slide.
If the hammer pushes off, or the gun is dropped while cocked and comes off the bevel of the trigger, both the hammer and the trigger return to their "at rest" positions. Due to the impetus from the rebound spring, the trigger and rebound slide assembly arrive ahead of the hammer, which places the hammer seat (on top of the rebound slide) in position to prevent the hammer from moving past it's "at rest" position, and the hammer nose (or the frame mounted firing pin) from entering and passing through the breechface.
Although the flag on the hammer block has been carried upward by the forward movement of the rebound slide, it is not touched by the hammer face in this particular scenario. (push off)
This mechanism, designed into the hammer and rebound works to prevent discharge of the gun, even if the hammer block has been removed.
The flag on the hammer block does serve to block the hammer's forward movement if the gun is dropped on the hammer spur while at rest, and the hammer stud were to fail. This is why the "hammerless" models like the 640, for instance, don't have hammer blocks installed. Also, this is another reason why the rebound spring should not be altered on service guns, as it plays a critical role in the successful operation of the safety devices built into each revolver.
The photos, of a de-milled 686, serve to illustrate this important design......
action at rest:
trigger back, hammer down position-firing:
hammer seat - rebound seat:
