OK I'm not a technically minded pilot like Nick!
The Makila 2A is protected from overspeed by dual FADEC and an additional back-up system. If all these fail and the engine still overspeeds the compressor blades shed. In the test the engine was oversped until the compressor blades shed. (No compressor blades: no more acceleration.) Nothing escaped the casing. This fully satisfied the EASA. This is thought by some clever people to be a worthwhile safety feature, but, it would appear, not by others. Fine! I guess they won't be using it in the future.
Happy Christmas all!