The video uses a common trope of "before and after" transformations, which is highly engaging. The humor comes from the implied cause of the transformation (steroid use) and the dramatic physical change in a relatively short period.
Summary
The video starts with a person saying "Nah man one cycle can't hurt." It then cuts to the same person 16 weeks later, appearing significantly more muscular and smiling.
Structure
1Initial statement about not being hurt by one cycle