I think it was more a case of Inoue's expert footwork and positioning just prior to unleashing that right hand. Rewatching the sequence, you can see Inoue pushing off his back (right) foot, moving his body forward and close to Fulton to deliver the hard jab to the body. Having closed the distance, and having forced Fulton to react to that hard jab downstairs, that makes Inoue's immediate right all the more powerful. This is one SCARY dude.