The idea of imagining your partner/crush with someone else in bed, and using the idea to spark an internal motivational rage.
The technique grants infinite rage to the user, allowing them to be filled by their self-made spite and hatred enough to be able to perform amazing feats, acting as a "fuel" caused by "cuck" thoughts (thus the name). As one of the hidden arts of the goon, the technique is most prevalent in gooners of the highest caliber -- one can usually spot a cuckfueled individual as a young male maxing out the machines at the gym.
Likewise, gooners of lower calibers are unable to comprehend the technique, calling it instead a "step closer to becoming a cuck".
Some say that the self-humiliation the technique brings is the main reason for the its effectiveness: by having an individual imagine their beloved/crush with someone bigger than themselves, their self-humiliation erupts into a demandful rage of one wish: "to be bigger and stronger than this hypothetical figure in their head", thus causing them to lift and exercise more.
"Cuckfuel actually works tho, transformed me into a beast in 3 months when I was thinking about her."