Thanks, I appreciate it! 
I actually found a pretty good Wiki on the subject titled… you guessed it - regenerative braking. It mostly refers to locomotives but the principles are so similar, much of it pertains to the Spark EV from what I could see. It talks about re-configuring the field connections on an asynchronous AC motor which draws a little energy from the permanent magnets. This energy is then used to "self-excite" some of the coils, creating a magnetic drag on the armature (braking) with excess juice channeled back to the batteries. This must be what happens when you shift into L and let off the accelerator. I'm not so sure software has anything to do with this, other than monitoring, since the degree of response seems so repetitive, like it's hard-wired into the motor controller.
As to what happens in D, I'm not so sure yet. Perhaps the coils are less excited or maybe it only involves fewer of them, or maybe the regenerated current is that which is freely spun off by the permanent magnets. In any case, software would still monitor and kick in whenever pressure is applied to the brake peddle, perhaps exciting coils up until the point when hydraulics kick in. I have noticed some variability in brake peddle response. In any case, it's fascinating for me and thanks to everyone for being so patient.
As to either L or D, all I know is that traffic is usually backed up in the morning and when I drive in L all the way, Li'l Sparky goes about 20 miles on 10 miles worth of juice. In the afternoons, there are periods where the speed exceeds 50mph, while in D, and the same 20 miles uses up 30 miles of juice. It really doesn't matter. At 10cents/kWh to charge at home, that's about 2 cents per mile, one fifth what I'm paying in gasoline.
Thanks again,
Dusty
