1. Used rarely to convey your 100% certainy of a situation or question. No backing out if you say 'yeah fuck yeah' to a quiery. Upmost form of agreement.
2. Also used in response to a phrase you agree with entirely.
Al: Wanna come round and put my girlfriend on the spit?
Me: Yeah fuck yeah. (Now I must go do it)
Me: Gravity hasn't found that ass yet, hot as.
Al: Yeah fuck yeah mate