Basically, the Z theory explains how modern-day 'Badmen' speak. If, when typing, the letter 'Z' is added to the end of the last word in a sentence, it automatically gives it that 'badman' sheen.
Man: Goodmorning, I are going shops for some bread and milk.
Woman: How illiterate.
Badman: Goodmorning, I are going shops for some bread and milkz.
Woman: Normally I would think that illiterate, but due to the use of the Z theory, I realise this chap is a badman.