When you shift into N, the hybrid system leaves the gas engine in whatever state it was in before shifting into N. If the engine was not running, it would stay off in N, so the hybrid battery would not be able to recharge when it got low. If the engine was running, it would stay on while in N and waste gas. At least that's how the HSD system works on the Prius, and I believe the RX400h system has the same design.