The spiral bits originated in milling and CNC work, where the bit was kept on top of the work as if it would be used on a handheld router. This means that an upcut bit will pull work towards the rotary motor, while a downcut bit will push work away from the rotary motor.
The downcut flush trim bit is ideal for cutting laminates and veneers because the bearing at the bottom of the bit moves along the substrate and the shear action of the bit pushes the laminates to be cut, producing an ultra-clean effect with no debris and chips. Imagine the cutting action of paper shears, where the base material is the workbench and the lower cutting position is the long cutlass of the shear.
The upcut router bit is ideal for use in a router table. Because of its geometry, the bit pulls the workpiece toward the router. This is also true in a handheld router. The upcut flush trim bit is a favorite for pattern routing at the router table with the template above the workpiece.
If you find yourself making projects with curves, highly figured woods, or you just need the cleanest cut possible, these solid-carbide spiral flush-trim router bits are a must-have.