Although widely known as a dance from 2007 created by Soulja Boy Tellem, the phrase also refers to male masturbation. The reason it is known as such is because the penis is similar to a solider with its helmet, and it is cranked like a crankshaft during masturbation.
"I walked in on dat nigga, and he was Crankin dat soulja boy, so I dipped."
Trill is when you are true and real to yourself. When you inhale the holy herb you reveal you to yourself. You become in touch with your true self. That is why we say we are trill or we need to get trill.