Circular saws should only be used to cut material that is on supports. A common safety hazard is to place a piece of wood between sawhorses and activate the circular saw blade. Unfortunately, as the blade cuts through, the wood will bow as the cut progresses. This can pinch the saw blade forcing the saw back towards you unexpectedly. This is called kickback, and also happens as a result of sawing with dull blades. The blade is pinched by the edges of the material rather than slicing through, and will come flying backwards at the person doing the cutting.