Great idea – emergency backup engine, plus power augmentation; both for short periods, but not really needed much longer.
A couple of claims are a bit outlandish, though, given typical temperature management of an aero engine (you just won’t taxi using electrics only and only turn the combustion engine on at the hold), and there will be next to no fuel saving as the main advantages of hybrid drives in car applications (running the engine at optimum RPM, regenerative braking) just don’t apply in aviation.
I wonder if it has a clutch to decouple the prop from the engine?
According to this article in a german magazine
it has a clutch, weighing 5 kg. On the downside of the system is certainly the additional mass of 40 kg including the batteries and the costs of 18700 EUR.