The pearl tramp stamp can mean one of two things. It can refer to the rubbing of a penis against the partner's butt cheeks as foreplay or in lieu of actual penetration. It also refers to ejaculating on the partner's lower back. Either way, it is possible to get some semen on one's lower back.
As a sex act, some view the pearl tramp stamp as a way of getting comfortable with the idea of anal sex. If the submissive partner is new to anal sex or does not feel at ease with trying it out, one can negotiate doing the pearl tramp stamp instead. Of course, try not to get carried away and respect the wishes of one's submissive partner.
