The blades are actually moving, its just the camera isn't filming at a high enough speed to capture it. If you look, the tail rotor is actually spinning.
this gif sucki sucki.
The helicopter turns it's engine off while in the air at a high altitude , then it turns the engine back on before it crashes
The camera is set to take a frame every time the blades do a rotation.
its a wierd illusion that cameras pick up while recording. don't really know how it works but i've done it and cameras just can't pick up the movement so it looks like they aren't moving at all.