Milton Vieira goes over 3 positions from half guard (top)
good times.
I've actually pulled that first oone off, rolling from top and getting the back, but I've found it to be a bit too risky to use as a regular part of the top game. Here is a link to Odie Neto teaching the exact same move with a slight variation from the Tools of the Trade 2 DVD.
http://youtube.com/watch?v=PFvdppqY5us
That move where you hit the armbar when you are getting calf crushed is really good to use even though the situation presents itself rarely.
what's the risk?
Yeah, the risk is that you can have your back taken. I'd rather stick to my 1/2 guard passes than risk it on a move than has real bad consequences if it doesn't work.
these kind of techniques are not the best to use in a competition :-)