Gets the heart racing when you read that title doesn't it?
Obviously we don't use a real knife for this - or even a dummy knife.
We use a felt tip pen.
This way you know when you're really getting cut and if your defence works. Then you can go away and drill specifics to eliminate where you went wrong.
Fighting with a pen is the ultimate pressure test of your knife skills - well, without doing it for real, which is not the place to find out of they work or not (especially "or not"!)
Many "pedestallers" find out the hard way that they really can't defend against a real knife attack by trying his sparring method.
Many don't even want to try it.