It primarily involves a closed fist contacting the soft underbelly of a person (beneath the rib cage) at a high velocity, causing the ensuing force to press upward on the victim's diaphram, leading to a sudden expulsion of air from the victim's mouth and lungs. This opening blow leaves the victim open to various other attacks, often leading to what would be called "bitch moves" becuase of the defenseless nature of the victim.
If two gay males are involved, and it is being used as a punishment, the term then becomes 'Sucker punch, gay revenge'. However, it is performed in the same way as mentioned above.
So Brett was totally going down on Nick and like totally went all Sucker punch gay revenge on him.