What is a hammer block safety and how does this differ from a transfer bar?

A hammer block safety is an actual selector switch that is a rounded pin with one flat side. When it is selected to the safe position the rounded side keeps the hammer from falling and hitting the firing pin. When placed on fire the flat side rotates around and allows the hammer to fall all the way forward to contact the firing pin. This is different from a transfer bar because it doesn’t require the trigger to activate and in my opinion is safer to use.