I slide right foot forward...
Drop/Angle your front ski, you literally grab hold of the surface edge of the rail or box by doing this, allowing to you spin the FUCK out of whatever you are sliding.
It's easier to see in person and pick up on.
I didn't read much of the comments so this might have been said already, but edging is very important.
When you're sliding a rail or a box, you will usually edge with your downhill ski. Drop you toe to spin blindside/uphill, drop your heel to spin frontside/downhill.
You can also edge to help keep you on the element.