When something is so funny you cock your head back and begin laughing. This is reserved for situations when LOL (laughing out loud) is just not enough and ROFL (rolling on the floor laughing) is just too much.
#1 - "when you said he's gotta cut his hair, what do you mean?"
#2 - "he's gonna bust out the flobee"
#1 - "HCBL"