2H swing you throw the bell forward with your hips, arms guiding; it floats at the top, gravity brings it back down, you lightly guide it into the hinge/backswing to swing again.
Shadow swing you throw the bell forward with your hips, arms guiding; you either let it float at the top OR arrest its ascent slightly before the float by using the lats, then by using the lats and a little extra tension in the arms you forcefully throw it down into the hinge/backswing to swing again. It will be a faster swing set than a regular 2H swing. You will feel it in the abs and lats more than anything. It's a great teacher of some advanced subtleties of the swing...