When in the L position when there is no more charge that the computer will send to the traction battery the negative split mode then kicks in and then uses compression braking on the ICE. Compression braking doesn't start until the traction battery has as much charge as is programmed to it.

When you brake in the L position the regen feature kicks in as mentioned above. The manuals also note that the rear brakes will be using friction braking. The front may also but the regen will divert some braking effort into the traction battery.

The Ford manuals note that the rear brake pads will wear twice as fast as the front brake pads because of this. As it turns out that's not quite what I saw. I use L for regen braking a lot.