Sounds sort of like, back in 1986 I started shooting with the Forest Hills Black Powder Brigade in Oregon, and the bourgeois said we could not rest our supporting arm against our side. He wanted to be able to put his fist between your elbow and your body, because that was required by the NMLRA. I still shoot unsupported, but from photos I see in Muzzle Blasts most folks have that left elbow (for right handed persons) firmly pressed against their sides. I should look if there is anything in the rules on that....
~Kees~