It's called "Retrograde Ejaculation." Basically, what Gary said. The semen is diverted to the bladder, where it will be eliminated when the man urinates. I wouldn't recommend this as a regular thing. A lot of doctors simply do NOT recommend causing oneself to experience retrograde ejaculation due to the risk of putting pressure on the pudendal nerve. This can cause numbness of the penis and sexual performance issues.
Some men have a problem with their urethra, or where the urethra connects to the vas deferens, (cum tube) where Retrograde Ejaculation happens regularly, and they eventually need surgery to correct the problem, especially if they want children, (because they have no appreciable semen expelled from the penis during intercourse) but also because it can cause long term health problems down the line. Diseases like diabetes, radiation or surgery of the testes or muscle disorders can cause the muscles which keep the neck of the bladder closed during ejaculation (so you don't pee while you orgasm) to become weak can also cause RE. It is not know if repeatedly inducing RE could cause bladder neck muscle weakness, but I wouldn't take a chance on it. Men who have clinical RE say their orgasms eventually become less strong with time without treatment.
The semen goes to the BLADDER (which is a holding organ) not the kidneys (which are filtering and excretory organs, much higher in the urinary tract, attached to the bladder by two VERY long tubes) If it was getting to your kidneys, not only would it be terribly painful, it would probably also cause immediate health problems.
