Ah I see yes the ones without a clutch and a blade will have problems. I repaired one where the user was using a tri arc blade and everytime you hit something solid it would oscillate until you shut it down due to the flex shaft and the lack of a clutch. It the weight of the attachmet that is the problem as it causes the flex shaft to twist torque.
Personally I wouldn't have brush cutter setup without a clutch; they just plain dangerous. Using a string trimmer head is not so bad but a saw blade is.
