kwsmith007, is actually pretty close on the overall design, and it was considered to be the weakest point in the entire link and caused some great headaches for a while.
in simplest terms, it is the dampner (center) portion of a manual clutch disk. the outer cage is bolted the engine flex plate and holds a series of springs connecting it to the center carrier w/ splines to link it to the generater.
the ICE operates in a series of pulses and the generater in series of phases you do need something here to smooth out the two. The ICE misfire detection comes from watching the firing pulses and the variance they cause in the engine rpm. becuase the generater moves in a series of pulses it self if there was a direct connection between the two, the ICE would not operate becuase the pulses from the generater would be mistaken as misfires due to the variance they cause in crankshaft speed. thus the need for a dampnend coupler. the occasional rattle type sounds is actually the springs in the coupler bottoming out and the engine and generater forces colliding. the engine and generater have to learn each others pulses for them to function, otherwise the ice gives up thinking it is misfiring (which kills the cat converter and the coupler in short order if left to occur). prior to 613.5 approx 40% fot he cars left the factory unlearned and took about 250 miles of operation to function. 613.5 updated the calibrations and allowed the two to start working in about 10 starts. 615 has smoothed things out further.
Now fisker is about the only one doing a spring dampner in this application and new deisgn is being considered, because of the liabilities it has. Ford and Toyota use a slipper clutch style coupler, GM uses a hydralic dampner (think self contained torque converter) and none of them have had issues with their setups.
As for what happened: very well could be the coupler failed, broken spring being my guess, or just as easily the ECM and GEN got into a fight over which pulse were which (thus speeding up if it was a load induced misfire) and the ICE finally saying, screw it i quit and shuting down. this triggering a loss of current when the HCM was requesting it and triggering a Powertrain fault. if the noise is present on the next startup, then it would be mechanical. if its quiet, then its software.