In my experience it behaves like a contact allergy. If semen gets into contact with my skin, the symptoms trigger. I tried storing some semen in a plastic bag for some days till I was sure that I would know if POIS was triggered again, and when I smeared some on my fingers the symptoms activated. Later I tried smearing some on my feet and that triggered it as well. Lastly I tried smearing it on my penis and that also activated it.
It seems, however, that the symptoms does not get triggered, when the semen is only in contact with the gland and the part of my dick that is normally covered by foreskin. This means that if I am very careful (and a bit lucky) I can masturbate and ejaculate without triggering POIS by using loads of paper towels.