Recently one of my intermediate students asked me how people pass my Guard. I explained to him that this pass was done to me frequently while I lived in Brazil and I absolutely hate having it done to me. It's quick, it's effective, it's easy, you look like a fool when it's done to you and, most importantly, it's VERY difficult to recover from once someone has stepped through.
With that being said there are a number of different setups that you can use for this pass. Basically any time you are standing and your opponent does not have direct control of your legs you can do this. After I showed it in class a few months back everyones been hitting it frequently (even on me).
This pass works both gi and no gi. It works when they are seated up and when their back is on the floor. If people are interested in seeing it done in no gi for when someone butt flops guard I can show it as well, just let me know if you're interested.
It is very important that your opponent does not have a grip on your legs. In the event that they do get a grip on your legs you will probably want to do something else.
When stepping through the gap make sure that you step with the top of your foot towards the belt and lead with the toes. This angles your hips and your shin to set up an effective pass on the other side. You dont want to land flat on your foot as it will make it difficult to make other transitions as needed. Make sure you land on the balls of your feet with your knee on the belly. This will be one of the few times that you will be able to maintain KoB after passing as it traps them with both of their feet off the floor.
The trapping of your opponent with both feet off the floor is important for the domination of this move. You achieve this by pulling up with the hand between your legs. As you practice this you will realize that it is odd feeling as there are not many other moves where you are standing pulling straight up into your own crotch but the angle will change greatly if you try to pull across your thigh. I refer to this pulling up and holding KoB as "peacocking". This is your oppertunity to smile and look good for the pictures.
Once you have achieved your points or your opponent is squirming so much that you cant maintain the grips anymore it's time to transition into the next position. You can now let go of the leg between your legs and immediately crossface to flatten your opponent back out.
You can do what I did in the competition and spin to the back. Any time that I start to pass the Guard standing I aknwolege that I am doing so without the underhook. You can take the gamble and try to keep them on their side and flatten them out with the shoulder in the face but the truth is as you get better it is VERY difficult to do this in a quick motion from standing to the floor. In this event (any time I dont have the underhook), I prefer to spin to the back.