That's because a bowed board often rocks back and forth on the saw table, so it's hard to make a controlled cut. Worse yet, the workpiece can pinch against the sides of the blade as you make a cut, causing a dangerous kickback. Fortunately, there's a simple trick that will help reduce the chance of this happening.
Start by placing the board on the saw table so the bow faces up (Detail a). Then make a couple of shallow passes (about a 1/4" deep), overlapping them to form one wide kerf. Now make a full-depth cut all the way through the board (Detail b).
As the saw blade cuts completely through, the board will "settle" a bit so it sits flat on the saw table. But it won't pinch the sides of the blade. The wide kerf provides the extra clearance that's needed to prevent the blade from binding. The end result is a safe, controlled cut.
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