Regen 'works' on the over-run - while freewheeling - the extra resistance acts as a brake so you will descend more slowly.
The problem is only a tiny amount of current is produced, and then only for the few seconds here and there while the bike is freewheeling.
Many, many times more current is used to run the motor on level ground, let alone up a hill.
Regen on ebikes is a pointless gimmick.
it's something people want to believe works and are encouraged to do so by the likes of KERS in F1.
But there's a huge difference between the amount of energy you can recover from hauling a heavy car down from 200+mph than there is from slowing bicycle from 12mph to 10mph.