Fixed bar path behind you means pure trap focus.
Stand close to the bar so it tracks up your back and doesn't drift away from you.
Holding the bar behind your glutes changes the pull line of the shrug, keeping the load tracking up your back so the traps lift straight against gravity without the forward drift a front-loaded shrug allows. The fixed vertical path means there is nothing to balance, just hang, shrug and squeeze. It earns its place as a strict trap isolator, particularly for lifters who feel front shrugs in their arms and shoulders more than their traps.
The bar behind you stops your shoulders rolling forward and keeps the pull line tight to your body, which many lifters find gives a cleaner, more direct trap contraction.
No. Straight up, hard squeeze, straight down. Rolling adds nothing for the traps and just grinds the shoulder joint through a pointless circle.
Close enough that the bar brushes up your glutes and back as it travels. Drift away from it and the fixed path starts dragging your arms backward instead of loading the traps.
