Seems like when Ford showed us it's Yellow "Tonka" truck a few years ago, they were talking about HLA (hydraulic launch assist). I forget where I read the article.

Basic operation was as follows.

When you used the brake it engaged a hydraulic pump. The "load" of this pump slowed you down by building psi in a nitrogen filled accumulator tank. Upwards to 30,000 psi best I remember.

When you accelerated, the tank discharged the stored psi back into that same pump thus providing free acceleration. I think the article stated the system would cut fuel usage by 20% in stop and go driving.

I think they also stated it could be a modular system that could be retrofitted on most vehicles.