Can be used to describe any action in which you do something awesome to the extreme, however should not be used lightly.
-"Yeah bro he's been shredding gnar all day in the pipe."
-"Bro I met this girl last night at K-train's place, took her back to my place and we shredded gnar all night."
How not to use:
"Bro I've been just hanging out all day, shredding gnar."
-"What? Bro you can't use it that way."
"Bro did you see me shred gnar on that blue groomer?"
-"Dude check yourself, you can't just throw verbiage around like that."