It's the unicorn mode that earlier quote said exists but WOT says does not.
Until someone actually records data of it happening, we have to assume it doesn't exist and it just stops regen.
If it does exist, the only way I can think of it working is that MGB is set to full regen (more than would normally be extracted from downhill coasting in L) and MGA is set to add that energy back to the system so that the electricity is passed from one to the other (@90%x90% efficiency) = 19% energy removed from the system as heat and out the coolant/radiator. At max regen of 60kW that's 11.4kW of deceleration possible.
That is what is rumoured to happen, but if that is physically possible, I don't know. But the numbers seem like a reasonable level of deceleration can be maintained from this.
So, I took a trip out west and descended the Coquihalla
with a full battery - for science.
I immediately regretted it when there was construction partway down the steepest descent (!) and regen cut to 1/5, but I believe I have data which supports the mythical battery-full-regen mode.
You'll see the computer allowed an absolute max of 86.6666% SOC, starting this mode about 85.88%.
The computer unhooked the two motors and brought MGA to zero. MGB continued a little mild regen until SOC hit 86.66% and then it flipped MGB into positive flow and spun up MGA into negative flow at a higher level (about 11-13kW difference).
-11kW was what I saw on the DIC while driving.
Once SOC dropped below 86% and net result of energy draw was positive (i.e. I was using a small amount of energy to drive as I had resumed driving @ 120 - thanks to air resistance slowing me that was not present @ 60)
So there you have it. Through the scientific method, I would say this does not disprove my hypothesis. Seems to show exactly what was described - one motor is running opposite to the other, and the net difference in energy pulled from the system would be the sum of the inefficiency of the process (i.e. heat expelled through the radiator)
Edit: looks like forum resized my attachment, but I think the visual is pretty clear as to what's happening. Yellow is MGA, blue is MGB and grey is net power. Dark blue is SOC and dotted red is max SOC mark (86.6666%). Green is vehicle speed.