The stop block is a piece of scrap wood about a foot long that you clamp to the extension table to set your standardized length. All you do is butt each spindle against the block and cut away. Just don't attempt to cut pieces so short that holding them puts your hand dangerously close to the blade. The real trick, however, is to cut a 45-degree angle into the end of the stop block, leaving only a ¼-inch flat spot to use as the stop. This prevents the sawdust from building up against the stop and causing spindles to be cut short.