So if we assume that stress vulnerability/susceptibility is a marker of vagal tone and people having a higher susceptibility out of POIS could mean that their vagal tone is out of normal equilibrium. If they are even more vulnerable to stress inside POIS could mean that they are shifting their vagal tone even more out of equilibrium. Sudden recovery from POIS symptoms is basically a reverse of vagal tone towards the out-of-POIS non-equilibrium value.
Ejaculation itself is induced by the SNS which could mean we are dealing with a sympathetic state. Some people mentioned that strong orgasm could ease their POIS symptoms a little bit. Orgasm itself might act as a stress reliever, activating the PSNS.
Graphical example where the only axis, the horizontal axis, depicts vagal tone (N= normal state):
-------N------(1)----------------------(2)-----
Out of POIS vagal tone is in an abnormal state (1). POIS pushes it from (1) to (2). This transition is basically the build up of POIS symptoms. Recovery from POIS symptoms could be a reverse transition from (2) to (1). The position of (1) and (2) might be variable as well depending on some unknown factors. The distance between (1) and (2) or between N and (1) might represent symptomatology and duration as well from person to person.