Aircraft engines running but not moving?

if engine tested on aircraft @ full power brief period, aircraft have brakes on , wheels in chocks. aircraft moored or tied down anchors longer duration tests reduce brake wear , safety. when tested out of aircraft, engines bolted test stands or test cells well-anchored foundation.

i have seen videos of piston , turbine engines being tested great power either aircraft not move or if engine mounted onto bench top (for example) looks blowing air. why engine not cause bodies accelerate?

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