A fingernail profile on a gouge is a different profile than a standard profile on the same chisel. This altered profile can allow for more detailed work with a spindle gouge and larger cuts with a bowl gouge.
Doing this on a tool rest against a grinder, it can be a daunting task, and the chance of multiple bevels is very high. However the Robert Sorby proedge fingernail profile kit profiling arm makes this process very quick, easy and repeatable by using this kit.
This boss is the steel block that the profiling arm fits into and mounts onto the metal bar in front of the grinder.