see if i can describe it for you.
1.While in mount, ride low and put your LEFT arm under his neck. (shift weight to right to avoid upa)
2.With your LEFT hand grab inside your own RIGHT sleeve (4 fingers in, thumb out)
3.Then bring your RIGHT 4-arm over his neck (your RIGHT palm should be palm down on your LEFT 4-arm)
4.To finish, rub your RIGHT palm down your LEFT 4-arm (like your brushing dust/dirt off your arm)
*The grip on step 2 is important, if you lose the choke it's probably b/c this grip is loose.
*If your rolled, don't worry, you can finish this from the guard.
I did one in a comp. about 6months ago here is the link.
www.coloradobjj.com/video/matt3.wmv
You can start the setup when inside someone's half guard.
I've finished it in someone's half guard, but the leverage is better from mount or guard.