‘Polarity’ is one of those jargon words that may confuse the electrically uninitiated. In its loosest sense, polarity means the direction in which the electricity flows. Your point blades may be pointing to one particular track, which would allow a train to take that route. But electricity doesn’t care about which way the point blades are going. If the frog is live, electricity will flow across it and out down both diverging tracks. It doesn’t take much imagination to think about the chaos that this could cause. There needs to be a way to ensure that the power flows out only along the correct track. You can rely on the point blade making contact with the stock rail to transfer power through the point. But if you’ve painted and ballasted the track then dirt can get in there, or the point’s spring mechanism can fail. To ensure reliable operation and to get over the issue of polarity, you can add a physical switch to the point’s polarity.