An extension cable is a piece of cable with loops in each end that is used to extend the snare in order to anchor it to a something farther away than the snare itself will reach. When anchoring to trees and brush make sure you tie off very low so the beaver can't chew down your anchoring system.
I will often anchor my beaver snares to rebar stakes that are from 12 to 24 inches long depending on how stiff the ground is. I will support my snare off of the stake as well. If I do this I don't use an extension cable. All my snares are swiveled.