Regen doesn't take place unless the vehicle is in motion so hill hold by regen is impossible. Yes, you can lock the rotors electrically but it takes stator current and that would waste energy.Id be curious to know which drive mode he was in and what the regen brake settings were. From other reviews, I've seen people stopped on descents I believe without having their foot on the brake. I'd imagine it would also allow for hill hold with max regen on. Or perhaps the weight and grade of the hill were more than the regen could.