This is masturbation for a special purpose - to test physical attraction to a prospective significant other. While masturbating, you think of the other person. If you are able to finish with only this mental stimulation, then you can be satisfied that you are attracted to the person physically, independent of their personality or their feelings towards you. If you cannot get off, then a relationship with the person will likely fail, as you likely will not find them sexually attractive.
Last night I took the relationship test with Meagan. It doesn't look like we're going to make it.