Kickback

Kickback happens when a workpiece binds against a spinning blade -- most often on a table saw -- and the blade's rear teeth, still moving upward and toward the operator, catch the material and fling it back violently. It's the most dangerous common failure mode in a woodshop, fast enough that there's often no time to react. A riving knife, a sharp blade, a properly aligned fence, and a push stick keeping hands clear of the blade's path are the standard defenses, and none of them are optional.