Ejaculation for men
Every natural sexual process for a man has to be ended with ejaculation or in other words - ejection of sperm from erected penis. When man is having ejaculation, it's also called "finished".
Eruption of semen fluid from penis is performed under big pressure and with kind of waves. If man manages to take out his penis before beginning of ejaculation (or women will drive him till ejaculation with touches), then putting the hand over penis she can feel strong ejaculation jerks. Ejaculation happens rhythmically, with certain intervals. This rhythm is not studied enough yet. Cause of these jerks is convulsive contractions of muscles of penis' cavernous bodies. After ejaculation penis is gradually fading and the more energetic ejaculation was (the stronger erection was), the quicker this process is.
Ejaculation is accompanied with feelings of pleasure (orgasm) and convulsive movements of body. In some way ejaculation affects length of intercourse. After eruption of sperm, men loose their lust right away, that is, they loose interest into continuing of intercourse. And since sexual excitement in man's nerves is gone - penis is going down and "falling asleep" right away.
Often woman is interested in her partner to have ejaculation as late as it's possible, so that penis would stay erected in vagina as long as it is necessary for her. To achieve that is possible with prolonged sexual intercourse or with a use of special men's "slowing down" cream.
Quick ejaculation after just few frictions, and often even right when entering penis into vagina, tells about too big sexual excitement of man's ejaculation centre. Such men are called partial impotents. Completely impotent are men who can't get their penis up at all or their erection is completely insufficient for intercourse.