The B-fly Guard as made famous by Sport Brazilian Jiu Jitsu was inherited from Judo. It can be a very powerful position because it allows you to sweep or submit your opponent while they are stuck playing defense.
As long as your opponent is defending they cannot strike effectively. If you stay close and do not allow them to create space you are safe.
Of course there are ways to counter and pass this guard style as with any guard. There will be both offensive and defensive techs in the Techniques Library below.
The transitions from this position are much faster no-gi due to the lower number of secure handles. This is due mainly to not being able to reach under your opponents arm and grab their belt from behind.
You have to control your opponent's upper body. If you don't your opponent will pass your guard. Since there is no gi or belt to grab the grips are different no-gi. You can easily use the same grips for no-gi when training with a gi.
If your opponent does start to pass your guard you have to stay ahead of them in the transitions. You want to get to half guard, full guard or stand up before they are able to pass.