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This is actual a legal ( by competion stardard) technique- As long as the glutes remain in contact w/ the bench an arched ( even severely) back is legal AS LONG AS the glutes remain in contact w/ bench. On the side view the glutes do indeed touch the bench during the entire lift.
Remember this is a competition and the goal is to create the shortest distance b/t lockout (notice her ultra wide grip) & the chest.
* ( On side view)-Notice that even after lift off ( but before the descent of the bar) she lowers her glutes to touch the bench.
* Also this is a 1RM, thats it! Not a 5 or 10 rep set like the tool in the first vid.
*granted this girl has hyper flexibility and most could never pull it off.
* with that being said I would never coach a non-competitor to use this technique