We waste energy in may ways going down hill unless we are recapturing it to recharge the traction battery or allowing the vehicle to gain speed (kinetic energy storage).

If we slow the vehicle down either by using "B" or using the friction brakes (not a light touch as this regenerates energy and recharges the battery - until it is full), we generate heat which is usually waste energy (on a cold day the heat generated by spinning the ICE - pardon the pun - may help keep you, the ICE, and the catalytic warm).

JeffD