its the part that goes into the hammer and forms the head onto the outside. the bucking bar / block is what is placed onto the inside and prevent the rivet slipping whilst being formed.
the snap is the forming end of the job
here are some old pics of my first snap prior to it being hardened (they are a black / blue colour after this process)
I have set around 400 rivets with my first snap and its still going strong, some on my project and some on other bits and bobs.
the tool shown is machined for guns with 3/8 shank tools.