While details are limited, Autocar is reporting that the XF features an advanced flywheel hybrid system (FHS). Currently undergoing devolvement, the FHS has a stop-start city mode which allows the flywheel to collect energy during braking and dispute it during start-up. When combined with an engine stop-start system, fuel consumption would be lowered by 20 percent.
Besides the extra efficiency, the FHS delivers an 80 hp boost for seven seconds. While that's not very long, it would come in handy during overtaking. Production seems feasible, but Jaguar hasn't made any announcements of yet.