The hug pass is one of the most versatile open guard passes. It can be used with or without a gi.  It can be used against most forms of open guard, and has a strong half guard pass built in, thus making it very practical against a skilled opponent.  The key thing throughout the pass is to constantly drive forward with your knees off the mat, keeping most of your weight on him.  Most counters that the bottom person can do involve creating space and pushing the top person away.   He can't create space by pushing the top person away because he has to push away the top person's weight and driving force of his legs.  He can't create space by moving himself away because the top person is constantly moving forward.

1. Randy is in Xander's butterfly guard.
 

2. Randy makes a quick step to his left to get a good angle, digs his right arm under Xander's left, and drives forward using his legs, with his knees off the mat, trying to put Xander's back flat on the mat.  To stay sitting up, Xander must bear all of Randy's weight, as well as the driving force of Randy's legs.

3. At the same time as he digs his right underhook, with his left hand, Randy is pinning down Xander's knee.  Xander is making it hard for Randy to drive forward by putting his foot in Randy's hip.  In the picture, Randy's right arm does not have the underhook deep enough and his knees should be off the mat.   That was just a mistake when we were taking the picture.

4. To get around Xander's foot posted on his hip, Randy moves his left leg back, removing the point of contact of Xander's foot on his hip.  He then takes a BIG step over Xander's leg.  Think of a dog peeing on a fire hydrant.  Because Xander's knee is pinned down, even if his foot was pointed straight up, he still cannot block Randy as long as Randy can lift his leg high enough.

5. Now he is in Xander's half guard.  He has the underhook and his head is tight on the ground next to Xander's head.  Note that Xander can not push Randy's head away strongly in this position.  It's important which side Randy's head is on.  In general, with most passes it would be more common for Randy to put his head on the far side of Xander's head.  For some people it feels unintuitive to put their head on the near side of the opponent's head.  Why does he put it on the near side with this pass?
a) It keeps his weight on the near side, so it's harder for Xander to roll him over.
b) It prevents Xander from going for a Kimura on Randy's left arm when Randy puts his left hand on Xander's knee (next step).
c) Xander is unable to push on Randy's face; only on the side of his head.
d) It prevents Xander from turning perpendicular to Randy.  In general, a good move from the half guard bottom would be to use his right arm to hook in Randy's left leg and spin his body so his right ear is touching Randy's left leg.  In this case, Randy's head is blocking that.

 6. Randy puts his hips up in the air and uses his left hand to keep Xander from pulling him back in the closed guard, and to push Xander's legs down so Randy can get his knee free.

7. Once his knee is free, he slides it over Xander's thigh onto the mat.

8. Xander traps Randy's ankle, preventing him from fully passing.

9. Randy uses his free foot as a lever to help free his trapped foot.  At this point, Xander may try to roll Randy over to Xander's left.  If this happens, Randy will just drop his hips down and stick his left leg out.  Since most of Randy's weight is on Xander's left side, it is not easy for Xander to roll over to his left.
What if Xander tries to grab both ankles between his legs?  To do that, he would have to let go for a moment to open his legs wider, and Randy could easily pass.

10. When he frees his ankle, Randy turns his chest down to side control and prepares to work a submission.