Sneaky forearm pump from behind your body.
Keep your arms and elbows still. Only the wrists move.
Holding the bar behind your body with an overhand grip and letting it roll to the fingertips loads the forearm flexors through a long range from an angle the seated version cannot copy. The standing position means no bench setup, just you, the bar and a grip that has to fight for every rep. It earns its place as a finishing move that builds the crushing half of grip strength.
Seated curls rest the forearms on your thighs with palms up, while this version hangs the bar behind you with arms straight, changing the pull angle and adding a bigger finger-roll component. Many lifters find it more comfortable on the wrists too.
About shoulder-width or slightly wider, wherever the bar can hang naturally against the back of your thighs without your shoulders straining. The grip should feel neutral so all the movement stays in the wrists and fingers.
Yes, that opening and reclosing of the hand is where a lot of the value lives, training the fingers as well as the wrists. Just keep it controlled, a dropped barbell behind your heels is a bad way to end a set.
