On select brush cutter models a rotary flail mower blade is available in place of of the standard "S-7" mulching disk. This 48 inch blade option is a heavy duty construction with free swinging tips that "flail" when impacting brush.
Flail = lower mass/higher RPM = better in smaller material
Disc = higher mass/lower RPM = better in larger material.
Flail blades are available on these brush cutter models: