Johnny explains where the transfer bar drag is located in the glock, what transfer bar drag looks like, why drag occurs, and how it can be remedied. This is addressed on his slide and frame prep video. However as this area is one of the major areas that creates drag in pull and return he wanted to make it more easily accessible for clients and Glock trigger enthusiasts. There is also other information on the Glock trigger bar specifically the Gen 4 bump on the vertical extension.
I'm a professional this video is only for education purposes DO NOT ATTEMPT TO DO ANYTHING ON THIS VIDEO!!!