It is used to check whether the contact surface is in even contact state.
In the past, I used Kohmiran or carbon paper, but even if they seemed to be hit evenly, clear differences were visible in this product. I think it is a very good product.
It was very easy to use and it was saved. However, pressure is applied also when cutting, the cut surface will be discolored, so it will be very helpful if you can improve it. (Was it not discolored if separating the paper separately ...?)