Invisible Collar Choke

I was looking at Eddie Bravo's website the other day and decided to try the Invisible Collar Choke. I couldn't get the move to work properly.My training partner said the move was uncomfortable but not enough to get a tap. He also managed to withstand the pressure on his neck by sticking his chest out. As a result he showed me a variation that worked much better and got him to tap.

With the variation i just grabbed his shoulder, instead of my instep, with my right arm (back of my hand facing me as opposed to the palm up grip in the last pic) and i locked my legs together but i didn't have his left arm in between his arm and head. I then straightened my legs and drove my forearm/elbow into his neck.

I'm gonna try it in class tomorrow while we roll and see what results i get.

If anybody knows what i did wrong with the original move some feedback would be greatly appreciated.