This is not a recommendation. Modifying your brakes can be dangerous.
On this Norton backplate, lining material has been removed from the trailing end of the trailing shoe, by a mad experimentalist.
It allows the cam to put more pressure on the leading shoe, instead of the cam being stopped from turning by the trailing shoe contacting the drum. That cam jamming effect is a characteristic of non-floating SLS brakes, and ones that float the wrong way.