[Lug_Nut]

Using L to brake is the same as using D and your foot pedal to brake, except it's completely different.
L has a fixed rate of regeneration created deceleration. That rate varies with the speed (more brake effort when the motor-generator is turning at high speed, diminishing as M-G rotation speed approaches zero) but that amount of effort is the same every time at that M-G rotation speed. You can increase the L brake force by also using the foot pedal, but you can't reduce the L brake effort, except by using the accelerator pedal.
In D your foot pedal can be used to infinitely vary the amount of regeneration and therefore the deceleration rate. You want less slowing force, lightly apply the pedal. More, use more pressure. Oh, and in the Gen1 the foot pedal operates the brake lights during deceleration.

How does the use of L increase regeneration? By causing to to drive at a slower average speed. By slowing at a faster rate when the accelerator pedal is lifted in L, more momentum is converted into recovered electricity than is recovered when the pedal is lifted while in D. That drops the vehicle speed more quickly and reduces the air drag to a lower level more quickly. It's that reduction in air drag that is the consumption reduction.
Driving at the same instantaneous speed whether in D or L will consume the same power and recover the same power.

regarding regen (either D or L) in low traction conditions:
I also shift to neutral when braking on snow or ice. Allowing the independent four channel hydraulic ABS to work to individually control each wheel permits more control under those conditions than an open loop, open differential, 'transmission brake' on two wheels could ever be expected to provide. Starting with hydraulic ABS also eliminates the pucker factor when the controller turns off the regen due to requested brake effort on one (or the other) front tire exceeding the coefficient of friction and slip-skid has started.

 

[Lug_Nut]

Under the low traction conditions there is less braking effort possible before the onset of wheel slip, so the loss of electric power recovery by NOT using regen at all on slippery roads is much less than the regen loss if the roads were grippy.
It's not that the car turns off the regen braking at wheel slip, it's the reaction time needed to respond to that complete loss of any moderate (regen only) braking the car was doing prior to hitting the ice, and applying more pressure/distance to the brake pedal to get hydraulic braking. That fraction of a second can be the difference.
When the roads have ice patches I'll very willingly trade a few Wh of battery charge for immediate ABS brakes on four wheels. My electric rates make the electron recovery savings far less than a replacement bumper cover and a radiator.
Dry pavement? Two wheeled braking by regeneration is frequently more than enough.

 

[Lug_Nut]

Regen doesn't 'release', it is simply that the motor/generator is spinning too slowly to produce the electric current at a rate necessary to continue deceleration at the driver's desired vehicle slowdown rate. Unlike friction brakes, the slowing force of regeneration diminishes as the vehicle slows.
There's also the onset of "idle creep" that GM uses to mimic an ICE with automatic transmission. At that speed threshold the motor/generator transitions from a regenerating deceleration device to a motor.
If the driver wishes to slow to below this 'idle speed', the brake pedal has to be pushed into the hydraulic friction brake actuation range.

Gen1 do not activate the brake lights regardless of regeneration created deceleration rate. Gen2 apparently do, and I wouldn't be at all surprised to learn there's a brake light switch built into the paddle operated regen on those. Could someone with paddle regen in their Gen2 check if the brake lights come on by simply pulling the paddle when stopped, in D, and foot off the brake pedal (like pulled up to a wheel chock, diagonal parked at a curb, parking lot curbing)? That would demonstrate that it is not deceleration rate that activates the Gen2 brake lamps in regen, but that there is a means, one not related to deceleration rate, that activates them.

Me? "L" mode has no purpose for existence and is a useless wasting of internet bandwidth.
The foot brake has a 'nearly' infinite range of regeneration capacity, "L" has a single range. If the driver wants more or less deceleration than the mandated "L" level he/she uses the pedals to vary that amount: Accelerator to reduce the amount, brake pedal for more. Just like "D"
L does not increase range. The power consumption of motor operation is exactly the same when accelerating or driving at constant load. L does not increase regeneration electric power recovery any more than applying the foot brake to the same level. Driving down a hill at some speed in L recovers no more power than driving down that same hill using the brake pedal to hold that same speed. Decelerating at the L defined rate recovers exactly the power recovered in D and using the foot brake to decelerate at that same rate.

L does not increase torque/pulling power. There is no "low" gear ratio that trades away mph to gain ft*lb.

L, in the Volt, is bordering on Chindogu. The issue preventing it from meeting the requirement of true Chingodu is that its uselessness is not recognized universally. I'm working on that....

 

[Lug_Nut]

"L" may be beyond useless... I would contend that it actually reduces your range/efficiency since it is needlessly regenerates on slight downhills i.e. your car starts picking up speed on a slight downhill, so you lift off the accelerator. The car will regen in L wasting your momentum. In D, the would regen far less and maybe allow your speed to increase slightly, but it would maintain most of the potential energy without needlessly (and wastefully) converting it to electrical energy to be stored in the battery only to be transformed once again when the road inevitably flattens out or starts up the next hill...

And that's the sole set of conditions that the regen actually DOES work to increase range. Whether brake pedal, or paddle, or L, with or without "paeddle", the lowering of road speed also reduces air drag. You can continue to 'coast' at an increasing speed and let the kinetic energy be squandered to create a bigger breeze, or you can slow down, have an easier time pushing air aside and 'bank' that air drag reduction as more regenerated electrons.
The gain in energy collected by going slower down the hill (more electric potential and less kinetic momentum energy gain, AND less loss to air drag) will get further up the next climb than will a faster down hill drive (less potential recovery, more kinetic momentum, and much greater air drag loss).