Milking is a topic that frequently comes up with D/s relationships when enforced chastity or orgasm denial is in play but special attention is paid to prostate health. As the prostate fills with semen continuously over a period of time without being emptied there are a few things to keep in mind.
If it stays in this full state for too long there can be a toxin buildup that contributes to prostate cancer. An overly full prostate can also cause swelling and some mild bladder control issues. It's also believed that prostate issues can lead to urinary tract issues as well. Basically, it's wise to regularly flush out the prostate in the spirit of good health.
The downside of this is that most subs (myself included) tend to turn into un-affectionate logs immediately following orgasm and proceed into period of lost focus and inconsiderate behavior over the next day or two. The solutions to this conundrum are milking or ruined orgasms.
To adequately explain these techniques it's important to first understand what constitutes a male orgasm vs. ejaculation.
Ejaculation is the expulsion of semen from the prostate.
The male orgasm occurs due to intense muscle contractions follow by a rapid expulsion of semen from the prostate.
The keys to both techniques is to induce ejaculation without allowing for an orgasm. Men who have experienced milking or ruined orgasms generally revert immediately back to a sexually frustrated and attentive state almost immediately after ejaculation. In some cases I have known that some men can even become more affectionate and emotional than normal in its aftermath.
Milking can take a variety of forms. The most commonly used technique involves stimulation of the prostate through the anus. There are prostate massagers sold specifically for the purpose of prostate stimulation. If this massage is done gently and slowly the prostate should drain itself in a way that more closely resembles a weak urine stream than an explosive shot. Icing down the genitals during milking can further reduce the chance of any pleasure on his part during the milking process.
There is a second method of milking that I will not go into detail about since it has the potential to ruin the male body's ability to orgasm. My first Mistress and I experimented with this technique trying it once a day for a week. By the end of the week my ejaculations were like a weak urine stream, dribbling out without any force behind them. The problem is, when she tried to have me orgasm for real, it also ended up dribbling out without an orgasm. It took three weeks of trying to retrain my body to orgasm to cum with any amount of force behind it and for a while we were both terrified that I would never be able to orgasm again. If there is enough demand I will post the technique but not as a milking technique, more as a means to ruining a sub's ability to have an orgasm.
Ruined orgasms seem to have become a bit more popular these days. When I first entered the lifestyle there was very little information out there about these but over the past few years there's been a spike in popularity in the Femdom community when it comes to these. The most common technique used to ruin an orgasm works a bit different than milking by allowing the prostate to have its muscle contractions but preventing the rapid expulsion of semen. This can be done by slowly teasing the sub's penis repeatedly bringing him to the point of climax before stopping. Eventually his plumbing will get so backed up that he can't control it and have him announce that he can't hold it back any longer. When that happens you can firmly press a thumb on the underside of the penis at its base near where the scrotum meets the shaft. You will likely feel some convulsions going on that start rapid and strong and eventually slow down and get weaker. When they have noticeably weakened, start alternating between pressing and releasing for very quick intervals (holding pressure for longer periods than you let up for). This may take some experimentation to get it just right. If done correctly the semen will dribble out in weak spurts and he will not have any of the pleasurable feelings of orgasm. Afterward, his genitals will probably be very sensitive and he should be in a state closer to his frustrated, denied, and focused state than his normal post-orgasmic obedience crash.
Any of these are useful techniques for keeping the male plumbing in good health while avoiding the usual BS that men put forth after an orgasm.