Every time I demo the Portable Kettle Bell sandbag, I go over the PKB swing. This movement really shows whether you're using more of your hips or your arms during the Swing. You want to use your hips as the driving-force behind the swing. If you use your arms, the PKB will have a tendency to "roll" over the wrists and break-up the rhythm of your swing.
Be sure to practice this with no less than the minimum weight recommended for your PKB. The 20-30lb PKB should carry a minimum 20lbs of dry sand or be filled to 1/3 capacity. The 30-40lb PKB should be filled to a minimum of 30lb of dry sand or be filled to 1/3 capacity.