Snipping out large chunks of unrelated material isn't really a troll trademark, but doing a little "creative snipping" or "quote forging" is, such as something like this:
Poster A:You're stupid
Poster B:> You're stupid
Yeah, because you're such a genius, doofus. Duh!
Poster A:>> You're stupid
Glad you agree!
I.e. snipping to change the meaning of the posters post. Or even editing the quoted material, adding text to it to make it seem the poster is responding to something he didn't.
Or leaving out large chunks of context to make the quote seem to say something it doesn't without any context.