A spinning saw blade’s rotational force resists your effort to feed workpieces through a cut. Dado blades compound that resistance. The wider and taller you set the blade, the more feed pressure you need to apply to remove material. For these operations, you should install a featherboard alongside the cut to keep workpieces secured against the fence and moving forward. Since rabbeting or dadoing are almost always jointmaking tasks, the dimensions of the cutaway area are important. So it’s prudent to install a hold-down over the workpiece as well as beside it. A featherboard placed here will ensure a consistent cutting depth all along the workpiece by keeping stock pressed down firmly.
How do you satisfy the need for both hold-in and hold-down support? Well, you could install two individual featherboards, but Magswitch’s Pro Featherboard ($59 in 2010) with Vertical Featherboard Attachment ($16) provides both in one device. The Pro’s base contains a pair of strong 30mm magnets that engage or release a work surface by flipping two rotating knobs. The optional top featherboard fastens to it with extension arms, and it has a second set of knobs that adjust it to the workpiece height (up to about 2-1⁄2"). This could be particularly helpful if your table saw or router table fence doesn’t have a T-slot channel that allows for installing the upper featherboard.