PEM is a reference to PMM 2 and brings eight special powers to be used in different situations. When you start the game, you will have no powers yet, however, to use the powers you must activate it. When active, you'll see smoke on the CJ's head. Once active, you can't deactive it, unless you're arrested or killed.

See also the discussion topic http://www.gtaforums.com/index.php?showtopic=360440.

Enjoy!