The large handle on the domino fence encourages downward pressure. When the part to be mortised is relatively narrow (see picture) the possibility of the plunging face of the domino tilting is a real possibility. This is especially so when the fence is fully down. Using the Bigfoot stabilises the whole process and gives a solid engagement. Without the Bigfoot any tilt means that the mortise is not perpendicular to the registration surface leaving the joint unusable in many cases.