Re: How many practice Rokkyo?
Rokkyo is AKA "Hiji Osae". It is the "elbow crushing" technique someone above mentioned. That to which some practice as a reverse grip ikkyo is Gokyo or "Ude Nobashi" ==> "Arm Stretching".
There are various flavors of rokkyo, but to me the big difference is in the "Elbow crushing" flavor and the "lever the tricep in the arm pit" flavor. The latter, I think, captures the original intent better while the latter is friendlier to uke and is often easier to teach.