No need to siphon gas anymore.
Cars waste a lot of energy. You may think the mods on your sports car are yielding extra power on your car, but the truth of the matter is that only about 14% of the energy contained in gasoline is used to propel your car forward. The rest is lost through escaping heat, driveline losses, and more. Volvo wants to recapture some of that energy and use it to charge its electric cars. The power will come from kinetic energy of passing cars recaptured through hydraulics.
See the XC 90 get a full battery from Jeeps and Audis and remember that next time you refill your gas-guzzler.