I think I'm understanding what your saying. I'm about to try the earlier advise about loosening the half inch nut on a couple and see what happens.
Get it up in the air and have a look.
You will see the brake is working on the layshaft not the driving axel.
This locks the transmission, same way as a tailshaft brake works on a truck or the P position on an auto box.
The diff has 3 shafts, an engine shaft, and 2 axel shafts.
If you lock the engine shaft the wheels have to turn in opposite directions which is how the P position works on your car and why you should chock the wheels when jacking uo.
Try it if you have an Auto box in your car.
jack it up and you will fine the wheels turn is opposite directions when you try to turn one.