It's purpose is to filer higher frequency power line noise. It is connected on the input (primary) side of the power transformer between either the hot (black) or neutral (white) on the power line. The other end is connected to the chassis. There is a ground switch on the amp to select a connection to either the hot or neutral line.
If the ages and capacitor shorts completely, you could have the line voltage on the chassis. Touch a ground, such as another amp, and you can get a shock. Hence the name but the term is exaggerated. More often these caps will leak a bit and the shock is mild.
If you have an older amp that has this cap, have a tech check it out to ensure that it is functioning up to spec. Many people remove this capacitor when a three-conductor power cord is added to the amp. It isn't necessary though as the ground switch can still be used to minimize noise in the amp related to the power line. As I mentioned above, Type Y capacitors are available that are designed to be safer for power line filtering. They should, but not all amps have these type of caps installed. Techs have been known to throw in whatever they have available.
More details here
in the TB portaflex Wiki.