One minute you might be accelerating and the onboard voltage drops as the battery supplies most of electricity. Then, as you reach the crest of a hill and start engine-braking, the car frantically tries to convert all the available kinetic energy to electricity, raising the onboard voltage to quickly charge the battery.
Which in practice means that they do a very miserly job charging the battery and are a ton more sensitive to a battery being in less than tip top shape so you can expect your battery lifetime to go down.
But it's a "win" because they pushed the serp belt change outside of whatever interval the reviewers who calculate TCO care about and they saved .000003mph in the process.