Peter, the conventional blades are small, short, light, and pivoted at the blade plate. If one of them hits something it swings back, rather than having to cut through it or hurl it violently away. The bar blade is long, heavy, and not pivoted; it can only stop by stopping the engine, or by shearing the drive key. Such a violent blow is involved that it sometimes bends the crankshaft.
This does not mean that conventional blades cannot cause very severe lacerations if they hit a person, but they are unlikely to perform actual amputations of limbs, which a bar blade can perhaps do. It is a matter of the effective moment of inertia of the rotating object. The existence of a pivot close to the point of impact of the blade, means that a much smaller mass has to be stopped by whatever the blade hits. If it weren't for the pivots where the blade mounts on the blade plate, conventional blades would be just as dangerous as bar blades.
