I will try to explain this the best I can. I had a Uberti Cattleman .44 Magnum (ca. 1975) that had this type of safety, and this is on a Uberti Flat Top .45 Colt I bought more recently. Here is the hmmer in close-up:
The wedge of metal below the firing pin rotates around the pin visible on the hammer, and is spring loaded. When the hammer is drawn back to the first notch (the safety notch, the hammer being raised only a slight amount) the tip of the trigger, the sear, engages a lever in the hammer that pivots this wedge downward, so that is impinges between the hammer and frame of the revolver. So engaged, this block prevents any forward movement of the hammer, even if struck or dropped, thus is a hammer block. Skeeter Skelton once remarked he used a hammer handle trying to make a gun fire with this engaged and a loaded round under the hammer. The gun did not fire.
Yet, so engaged, a loaded revovler can be fired by merely earing back the hammer and pressing the trigger in normal single action firing.
As I've pointed out, some Ubertis have this safety device, others use the base pin "Swiss Safe" system.
Look up Uberti parts on Numrich's (Gun Parts Corp) site and the schematic, and parts list the hammer with or without the safety.
If a single action has a base pin hammer block (as did my Great Westerns), you engaged this block by going to half cock and pushing in the base pin to the second notch manually. This blocked the full travel of the hammer and stopped the firing pin contact with the primer.
This is the "Swiss Safe" system, developed by Hammerli for their Virginian imported by Interarms. Note this is a different revolver than the Virginan Dragoon.