Supercoiled DNA is the native confirmation found in vivo and occurs when extra twists are introduced into the double helix strand. People often compare the forms of DNA to rubber bands or telephone cords (I know some you must still remember phones with cords!). If you over twist a rubber band or telephone cord, coils stack up upon one another introducing tension. In the case of plasmid preps, this superhelical tension cannot be relieved because the ends of the plasmid are joined together. Supercoiled DNA migrates faster than predicted in an agarose gel due to its conformation. Supercoiled DNA is the desired species when isolating plasmid DNA.