My brother found this out while he was messing around. if a vehicle is moving slow enough, tap X to hijack, and at the same time toss a grenade. if done correctly, the grenade will land in the seat of the vehicle(ghost, warthog, wraith, or spectre) kill the driver and passengers immediately. if you mess up, the grenade will explode where it would have if you had not been hijacking.
*NOTE--This is easier to do in multiplayer, because computer AI is set to automatically detect you.