To further this discussion I have posted some photos om my Smith & Wesson New Model Number Three. The Beretta Laramie is a replica of the S&W New Model #3.
The New Model Number Three was the culmination of Top Break design for S&W. It was the most sophisticated of all the Top Break revolvers they made.
The New Model Number Three had a rebounding hammer. There was a spring that forced the hammer back as soon as the gun had fired. In this first photo I am forcing the hammer forward with my thumb, overcoming the rebound spring. This is the position the hammer would be in as the gun fired. Notice the notches at the base of the hammer. In this position, the sear can be seen. The sear is part of the trigger. With the hammer in this position, the sear is forced forward and is resting against the curve of the hammer.
Observe the position of the sear relative to the hammer notches in the following photos and also observe the clearance cut in the hammer.
The next photo shows what I call the 'at rest' position of the hammer. The rebound spring has forced the hammer back slightly and the sear has popped into a flat milled on the bottom of the hammer. With the hammer in this position the sear is wedged against the flat at the bottom of the hammer. The hammer cannot move without breaking the sear. The cylinder is still locked with the hammer in this position. Notice too that the clearance cut in the hammer has moved back slightly. More about that when we look at the latch.
In the next photo, the hammer has moved back to the half cock position. The sear is trapped in the half cock notch, much like the half cock notch on a Colt. The bolt has been withdrawn now, and the cylinder is free to rotate. The clearance cut in the hammer has moved back a little bit more.
Finally, the hammer is now at full cock. The sear has popped into the full cock notch and the gun is ready to fire when the trigger is pulled.
Now, let's take a look at the latch.
Here are two photos of the latch itself. The gun is only partially open, so the hammer can be seen. Notice the relief cut in the hammer and the small shelf at the bottom of the latch. Also notice that the entire area above the shelf has been machined away.
OK, let's start to put it all together. In this next photo the hammer is at the 'at rest' position. The gun has fired and the rebound spring has pushed the hammer back to the 'at rest' position. But notice that the shelf in the latch is still partially obscured by the hammer lip above the clearance cut. With the hammer in this position, the latch cannot be opened because the hammer prevents it from rotating up.
In this next photo the hammer has been pulled back to the half cock position. Now the latch can be rotated up to break the gun open. The cylinder is also free now to rotate. This is the position the hammer must be placed in to load and unload the gun.
OK, just a few side views to complete the explanation. In the next photo, I am again forcing the hammer forward with my thumb, as if the gun has just fired. The hammer has not yet rebounded, it is all the way forward. There is enough clearance around the hammer and its clearance cut for the firing pin to extend all the way forward to fire a cartridge.
In this photo, the hammer has rebounded to the 'at rest' position. Notice there is not enough clearance for the for the latch to be operated. The hook at the top of the hammer will interfere with the shelf at the bottom of the latch if the shooter attempts to open the latch at this time. By the way, it took me a while to figure this part out.
Finally, the hammer has been pulled back to the half cock position. The cylinder is now free to rotate. There is now enough clearance between the shelf in the latch and the hook in the hammer so that the latch can be opened and the gun broken open for loading and unloading.