PSE burst advices?

Can someone explain to me properly and in proper details how the sweet mother of Lucifer that mechanic works? I get the gist of it but I cannot seem to pull it off when I want it most of the time..

Killing everything and when there's a rainbow top right where you PSE level are ups you can try force it by popping the mag on a mob pack encounter.

I recommend setting yourself up in a corner to min max those.