I received a PM about this a month or so ago and this is the reply I gave:
Question: “I was wondering, does a muscle contraction actually cause a reverse kegal or is it just the relaxation of the BC muscle? I'm in kinesiology class and although I'll never bring it up in class, I'm wondering if you know of (if any) muscle that is the antagonist (opposing muscle) of the bulbocavernosus.
After reading a bit more, I saw that you posted that a reverse kegel means to just relax the BC muscle. I always thought the act of trying to push your urine out harder was a reverse kegal and would seem like you are contracting some type of muscle since in the body, it's really hard to actually force a muscle to relax in a split second. Thoughts??”
Answer: There was a thread about this not long ago and I was going to write up a little history of the so-called “reverse kegel” here at Thunder’s Place. But I never got around to it. One of our members, drgmerlin, was one of the first to discuss it. He used it as a means to avoid ejaculation. Or at least the spasms of ejaculation. He still came, but it flowed out rather than in spurts. He felt that by consciously counteracting the natural muscle contractions he was able to prolong his erection and continue to have sex. I don’t remember if it worked all that well, but I do remember he was the one talking about it.
DLD then began to use it as a means to enhance manual stretches. Newbies usually complain about the “tug back” (yes, like Bib’s LOT theory) that their penis does when they first try to do manual stretches. That’s a normal penile reaction to being pulled on, it tries to pull back against the external force. As you know it’s the bulbocavernosus (BC, but also known, and actually better named, as the bulbospongiosus) muscle that’s doing the tugging back. DLD saw the reverse kegel, which as you noted is just relaxing the BC, as a means to make the manual stretch more effective.
It was one of them, or perhaps someone else, who said if a kegel is like stopping urine flow, then a reverse kegel is like forcing urine out. That’s not actually true, of course. You don’t have to force anything, but you do have to concentrate on relaxing the BC. The abdominals are not really involved nor is the urinary bladder or increased intra-abdominal pressure which are the actual muscles used to force urine out.
There is no opposing muscle to the BC. The reverse kegel is trying to not let it contract at times when it normally wants to, such as during ejaculation or when the penis is being pulled on. Doing reverse kegels as an exercise in and of itself is pointless. Unless you’re trying to prevent the BC from normally contracting, forcing it to relax won’t accomplish anything.