A moment of extreme triumph, or celebration, over the unlikeliest of odds, involving a 'heroic stance', firmly planted legs, hands either confidently on hips or perhaps held skyward, perhaps with a sword.
Derived, as far as I can tell, from a misunderstanding/evolution of the meaning of 'an achilles heel', as the knee bone is a vunerable part of the human body, but a part which carries the weight of expectation that such a pose implies.
"The bit in 'Born to Run' by Bruce Springsteen right after he shouts '1...2...3...4!' is a pure Kneebone Moment"
"Standing on top of a mountain with a broadsword raised to the heavens as thunder explodes around you is about as unadulterated a Kneebone Moment as you can get"
Any song written by Jim Steinman will probably contain at least one Kneebone Moment.