Do NOT do this as a habit. There is a little valve inside, that basically directs what goes down your urethra, semen or urine. Think ot is as a kind of check valve. You are forcing the pressure to go the wrong way, and it stresses the valve. It can lead to permanent damage and is not healthy. If you are concerned about a mess, then put a loth over the end of your dick, but don;t make a habit out of retrograde ejaculation!