From the Line 6 JTV-59 Pilot's Guide:
Note: It is also possible to supply power to your Variax without a battery. When connected to VDI digital input-equipped hardware, such as a Line 6 Helix or POD HD, power is supplied via this connection. You can also power the guitar through the 1/4 inch Analog Output jack using the Line 6 XPS A/B Power supply and 1/4 inch TRS cable in the Variax Cable Power Kit (P/N: 98-034-0105).