Marcelo Garcia demonstrates a slick B-Fly Guard Pass here in this video. I really like this pass.
Marcelo does the pass by kicking his legs high up into the air as his legs go by his opponent's hooks. This gives the technique a certain flare that is eye catching.
The technique can also be done with less altitude. It is quicker when you don't kick your legs up so high and safer.
One of the defenses from the bottom when someone does this pass is to push your opponent's head to the same side that their body is going and hipping out to the opposite direction.
If your legs are up in the air when someone does this defense you could throw your back out.