When the Justin Timberlake look-a-like starts singing, before the underwear model throws the dirty underwear at him he is standing up straight. When the dirty underwear is thrown at him, he turns sideways and starts to kneel down. In the next shot, he is standing up straight and facing forward again
(at around 1 min) You can see Devin Crittenden wearing a pair of yellow shorts with his legs shown. After Crista Flanagan hit him with a guitar, it was obvious that they used a double, because the legs don't match.