The idea is to allow the backing plate to rotate, so, the center has to be bored out and a spacer/center is slipped onto the axle with enough clearance to allow it to move freely. The torque arm is then tied to the frame rather than the swingarm.
As the rear brake is applied, the braking force creates a downward force at the rear end which helps to keep the back end down and braking.
It doesn't do much if the rear end is in the air, but right up to the pint of a stoppie, it helps. It's not easy to see but if you look carefully, you can see a steel spacer between the brake plate and the alloy spacer. That's a sleeve inside the backing plate and allows the plate a small enough movement to pivot easily and it also allows it to self center.
The first picture is on a CB160 racer and the other two are a CB77 350 race bike. Same idea though. That sleeve can be straight (without the end flange) if it's large enough, but if teh wheel spacer OD is much larger than the backing plate center, a top hat works better.