"Energy constantly flows through the crystal. You're not fighting with a simple blade as much as you are directing a current of power." ―Sabine Wren and Kanan Jarrus on the Darksaber, Rebels: s3e15 Trials of the Dark Saber

I'm sure there are other references, too, but for it to be a power current it needs to flow to and from a terminus. See also the Tagge Protoblade in which the energy circles around a metal core (internally referred to as the corn dog lightsaber) instead of being a sword