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.
Look around your gym. Most trainees lack the calf thickness to suspend a pair of double striped knee socks, let along balance out the size and symmetry of the lower leg. Now look in the mirror. Are you one of them?