Thanks for all the helpful advices.
Actually, before I come to yoga practices, I tried abstinence a few times (when I was a Christian many years ago). And I always try to due with sex with moderation later on. What I find out was: once I allow myself to masturbate, the desire keeps coming instead of fade away. So these years, I seldom have moderation: sometimes I don't have any such desire for one or two weeks, sometimes I have orgasm for many consecutive days--very unhealth in any standard. It just seems that an orgasm can destroy my will power and make me feel uneasy, at least momentarily. (That's before I do meditation. Hope things would change......) that's why I try to avoid it all together as far as I could to prevent that. (though I know I will fail sooner or later......)
Also, I am not sure if it's just a coincidence-- I tried sitting on siddhasana for a short time and seemingly it ignited my sexual urge (not immediately, though). Originally I do that in attempt to prevent the urge--exactly the opposite.
I want to try tantric masturbation, but still hesitate. I would like to make sure about the techniques and all possible side-effects before I do anything on such sensitive organs. It would be better if I could see some pictures. I learn from yoga texts that perineum is the area between the genitals and the annus, the pressing point in Siddhasana. But "Blocking can be done by reaching to the perineum from either the front or the back" according to lessons. Are we actually pressing the same area, and "front or back" refers only to whether my hand are in front of/behind my leg??
Also, what feeling should I expect from an orgasm with a successful blocking, comparing with one without blocking? There is some pressure building up inside.... Would it be painful? And will it weaken the power of ejaculation in the long run?