Not long ago in a different thread discussing Mountain Mode, scottf200, one of this forum’s moderators, posted a response from GM that originally appeared in 2012 that might help answer your question. I’m not good at posting links, so I’ll just quote the citation. Of course, this is speaking of the Gen 1 Volt:
" 02-01-2012 #18 Chevrolet Customer Svc
Join Date Aug 2010
Location Detroit, MI
You have come across a feature of the vehicle which protects the battery from overcharging. Under certain circumstances, the electric motors will resist one another to provide braking in addition to the friction brakes on the vehicle. In order to meet emission requirements, the Volt does not spin the engine, but uses clutch 2 in the drive unit to link both motors. When the vehicle is at low speeds, clutch 2 requires that the resultant planetary gearset speeds increase to compensate. You will hear the electric motors at higher speeds, which is certainly a change from their normally silent operation.
The Volt was validated using the steepest, longest descent in the nation, Pike's Peak. With a full battery, the volt can descend Pike's Peak without issue with a combination of friction brakes and the electric motors. This is part of the Voltec propulsion system which has many more delighting features waiting for you to discover.
Volt Advisor Trevor
Chevrolet Volt Advisor Team
...note that by counting it as Gas Miles, you artfully increase your MPGcs (gas mileage when using gas) by increasing your total Gas Miles without increasing Total Gas Used... so... bug/schmug... would you prefer that it artificially inflate your AER and ev mileage by counting it as Electric Miles?