Go wide to bias the inner bicep.
Keep your elbows fixed and your wrists neutral, no swinging.
Taking your grip wider than shoulder-width changes the angle your biceps work from, biasing the inner portion of the muscle that a standard grip underuses. The bar keeps both arms honest, forcing them to move together with elbows pinned, and the wide underhand position makes the squeeze at the top land differently to any other curl in your rotation. It slots in as a variation to run when standard barbell curls have gone stale.
The angle of the pull, which shifts emphasis toward the inner part of the biceps. It is not a different muscle, just a different bias, and rotating grips over time covers the biceps more completely.
Narrow the grip slightly until it is comfortable, the bias does not need an extreme width. Some lifters also find easing pressure by keeping the wrists dead neutral, as the cue says, solves it outright.
A few inches beyond shoulder-width per hand is plenty. Extreme width strains the wrists and shortens the range without adding anything, so find the widest position that still feels strong.
