I am curious if that HP boost needs to be used immediately or is it stored energy?
It’s there as long as the flywheel spins. But that system is very specialized — just meant to store energy from braking for long enough to use it when the officials turn on the green light.
You can find lots of links on a search of KERS.
My own electric motor scooter actually uses regenerative braking to store energy — but as electrical energy in the batteries, not as mechanical energy.