Turning the blade around backward is an old wive's tale that unfortunately won't die. It comes about from using the wrong blade on miter saws, but it has morphed into always doing it for all vinyl and aluminum siding regardless what type of saw or blade is used. It gets repeated without proper application, and that is what is so dangerous about it.
You should never turn a carbide tipped blade backward for any reason. In the event you feel the need to turn any blade backward, it is an absolute sign that you are using the wrong blade for the task. It is effectively turning a positive hook angle blade into the more proper negative hook angle blade that should be used. It is also a sign that your blade depth is too deep. That's why it originally began with mitersaws, because they don't have the effective hold-down that a circular saw has between the workpiece and the center of the blade. The aggressiveness of a positive hook blade will pull a thin workpiece toward the center of the blade, and for a mitersaw this will result in a sudden explosion of the workpiece as it gets grabbed and pulled upward.
As for circular saws, it should never be necessary....Ever. Even when using the wrong blade, it means your blade depth is too deep.
It should also never be done with carbide tipped blades. That is just asking for a projectile to come loose.
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