Long time in past the khurra was very famous while doing waterproofing, i too havent used this but got to learn about it from my one of the boss.
Khurra is a tin sheet which is installed at the drain location and hole is made inside of it, from the hole the drain pipe is installed in the cutout area.
Due to making hole in the tin sheet (take a paper in your hand and make hole in it by punching the pen at center) it gets more area to interlock and the surface of the tin shit does not allow the water to pass from it which prevents leakages on external surface of the wall.
Now a days you can see a MS pipes with flanges which is installed in RCC structure (water tanks) uses the same principle, does not allow the water to pass directly to outer face.
To imagine how khurra is, you can see how flanged pipe is installed in water structure, same way the tin sheet becomes a flange and you run a PVC drain pipe through it in cutout area.