Backer Board

A backer board is a sacrificial piece of scrap placed directly behind a workpiece at the point a blade or bit exits the cut, giving the wood fibers on that exit side something solid to shear against rather than nothing, which is exactly what causes tearout. It does the same fundamental job as a zero-clearance insert does on a table saw, applied more generally -- clamped behind a workpiece on a drill press to back up the exit side of a drilled hole, or behind a board being crosscut on a miter saw to support the cut's far edge.