When you turn your hip correctly your "opening" your hip and thereby get more reach, that's why you hit with a higher part of your shinbone. So with this different reach, you want to adjust the distance with your footwork.
You don't want to move your target (unless you want to create distance) as that's a waste of energy. You want the energy of your strike to be transformed into deformation energy in the target, because that's what inflicts damage.
This pretty much covers it. The bag moving when you kick it should be the result of the mass of the bag unable deform any further and the kinetic energy translates to motion through the chain.
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. Thatís it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit