When driving in the mountains, what is the top speed at which you may down shift from drive to low in order to use the regenerative action rather than braking to control speed?

This might be addressed elsewhere, but I can't find it. When you shift from "D" to "L" (or vice versa) what exactly is the change? I assume it's a mechanical change of some kind (as opposed to an electrical change) since there is a noticable change in drag when shifting to "L" while coasting.

Hi there, I am not sure if this has already been suggested. But as a 2012 Volt owner who will most likely buy the next gen Volt I would like to suggest the following. I switch back and forth between "D" and "L" to recover energy and reduce break use. It will be neat to have a button on the...