[johnsmith182]

you drop your shoulders and round your upper back a bit at the start, but not the lower back. then in one explosive movement, you pull the shoulders back, "scrunch" or arch your upper back, and pull with your arms. it all happens at once, and the bar hits you hard in middle of the abs. this is not a slow exercise, its a fast and powerfull contraction of all the muscles of the back, which causes all 3 things to happen simultaneously. different people look a little differently, depending on their mobility in those parts of their body. most people can drop the shoulders and round out the upper back more than this kid. no matter how mobile you are in these areas, it is the EFFORT you make to do all 3 of these actions at once, and quickly and powerfully that makes it work really well.