The punch appeared to be under the beltline when it was approaching the short pants of Usyk, however, when the punch touched Usyk it is certainly not under the beltline. It was on the beltline. I will not be shocked if this punch has given Usyk a big profusion of low bowel movement after the fight.
Also, @pawel7777 is correct. This only happened because the abdominal muscles were relaxed. This might have occurred while Usyk was inhaling. Tyson Fury was trying to repeat it, however, Usyk's abdomen was always ready and flexing heheheheh.
In any case, is everyone excited for this rematch? I am not anymore. It might be more exciting to witness Bivol go to heavyweight and challenge Usyk if Bivol is victorious against Beterbiev.
For me, the punch was legal, Usyk was hurt, and he was saved by Pabon. If he wasn't given sufficient time to recover from that beltline blow, Dubois would have won via knockout.
Dubois is a gifted puncher. Whether relaxed or not, a perfect strong blow to the abdomen like that would hurt any boxer.
Bivol to the heavyweight? That's quite a leap. He only weighed 175 lbs. in his last fight while Usyk was more than 230 lbs. against Fury. He could climb to cruiserweight though if he feels like he is having a hard time making 175.