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 is the profiling arm that holds the chisel, and allows the chisel to rest against the grinding belt, and rotate at the same time giving the desired profile.