Its "Displacement on Demand" V-16's Active Fuel Management system shuts down 12 of its 16 cylinders during normal motoring and cuts eight cylinders during more spirited driving.
You'd think with Cadillac's failure of the 4,6,8 engine management system of the 80's they would shy away from this technology. My mother had 3 of these cars in the 80's and they were all crap. Cadillac = Luxury Crap, with no resale/trade in value. Why she kept buying them made no sense to me.