The B-fly Guard or Seated Guard can be a very difficult guard style to pass, especially in No-Gi Grappling.
Without the grips that gi pants offer, you have to use good body mechanics, technique and your body weight to pass your opponent's guard.
In the video below ADCC champion and Brazilian Jiu Jitsu Master, Jean Jacques Machado demonstrates a simple but solid guard pass.
What really makes this pass work is proper body mechanics and the correct use of your body weight. Once you have your opponent's hip blocked your chances of passing or going to the mount are very high.