roger lance
The marking knife will not be the cause of the tail board moving..... more than likely you (the craftsman....the maker) will be the cause.....you don't have perfect muscle control.....perfect balance......and the likelihood that you will (eventually) not be able to complete a large, long mark out of a sizable panel before you inadvertingly shift it slightly is high.

No one is telling you to include this feature....or any other feature for that matter.....make it your way.....as large or small as you wish.....as fancy or plain as you want.

I enjoy watching YouTube videos of Ishitani Furniture.....he doesn't use a Moxon.....but, when he is marking out his pin boards, he has everything well clamped down.....there is no chance for any movement of his tail board.....and he is taking his time to carefully mark out his work....and he is using both hands to do this.....I guess we could be critical of him.....or maybe learn something.