A simple rotation of the sandbag (or kettlebell, if no sandbag is available) while descending into a reverse lunge will challenge your strength and core stability in the transverse plane.
When you add in the power of a swing, what you wind up with is a really cool explosive movement – the swing requires decelerating the implement at the bottom of the movement before you explode from the lead leg into hip and knee extension.
Charles Poliquin is a genius when it comes to devising innovative, painful exercises and his One-Arm Barbell Eccentric Curls are no exception. They're guaranteed to hurt, along with eliciting stares from other gym members.