BFU report: "Without forward motion, with the fuselage nose lifted approx. 10° and a power of FLI 6.5, the helicopter first yawed a few degrees to the left and then suddenly accelerated to the right. In the initial rotation around the vertical axis, the fuselage nose raised to approx. -20° within a 90° turn to the right. At the same time the engine power was increased to FLI 10.3. The pilot in command tried to control the position in space by pedal, collective and cyclic control inputs.

Specifically, the change in collective control had the effect of increasing or decreasing power. Within approx. 12 s the helicopter completed approx. 6.5 revolutions around the vertical axis. At approximately 10:20 a.m. it struck with an inclination to the left on the grass surface.
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Both pilots were seriously injured on impact and the helicopter was badly damaged."

Published report: https://www.bfu-web.de/DE/Publikationen/Untersuchungsberichte/2023/Bericht_22-0060-3X_EC135_Heidehoefe.pdf