your thumb should be facing up and your knuckles should be down... i know this is not what you were asking for but this really helped me with my stance and my posture the whole lot of it.

Stand in your fighting stance, get close to the bag and "scoop" your fists into the bag in a slow exagerated motion( moving your hips and feet).

do you kinda get it now? i hope this is better explained.

good luck