When you see a guy getting wanked off for his sperm then quite simply this is male forced milking. Many men like the experience having been forcefully milked which is a real turn on for people that enjoy the sensation of hand to cock touching. This is not the only way to do the make forced milking but in one sense is the traditional way.
When you are getting milked by a woman or by a man then your penis is being rubbed and pulled until you reach arousal. This way in any fashion of getting the guy to shoot his load is milking the male of his sperm and therefore making him orgasm and ejaculates which is basically the same thing for the majority of men.
For the most pleasurable experience in milking you need to get your girlfriend or lover to trim their nails so that no nasty accidents happen with the end results being cuts and bruises to your dick. As all men know this can be very painful so it is a good idea to stay away from rough sex if you have sensitive or easily bruised skin.
This kind of sexual act is generally done when there is a dominatrix involved so that the man is either tied down or restrained in some way so that the master can milk him for his sperm. What happens next is up to the master in the game unless you can scream out your safe