a performance of some kind - music, drama, dance, martial arts, whatever - You go have a shit. Why? to get rid of excess weight and so you don't shit yourself
A PGS is considered a social event and many PGS-ers can go together to engage in humerous
banter and to describe the size, shape
and consistancy of the shit. You have to be pretty open and easygoing to be a true PGS-er.