A saw blade sinking through the face of a board can "walk" back across the surface, so make sure that no part of your body or the cord is in line with blade. Release the lever for the depth setting and drop the shoe below the blade. Then tighten the lever slightly to keep the blade from dropping, but don't lock it all the way. Bring the saw up to full speed, lift the guard, and slowly push the body of the saw down to start the cut.
Tip: Make sure to start back far enough so that you only push the saw forward; never drag a spinning saw back.
*starting the cut in the middle of a board