It's not a shrug... You need to retract your scapulae. Think about rolling your shoulders back like you would when you puff your chest out. Take a breath in, hold it, flex your lats, keep your elbows close and work on exploding off your chest. Keep your feet planted and try to use them to transfer energy from your lower body. Do all this correctly, and you certainly won't be benching less than you would with a flat back/flailing legs, etc.