This post is dedicated to everyone who has written an article on a model railway and included a phrase along the lines of "I don't understand the wiring..."
Here we see how the polarity of the point frogs is changed. The black wire goes from the common to the frog. The red wires go to the stock rails either side of the point. Thus the frog takes the polarity of whichever rail the operating rod is nearest.
I know this could be a bit slicker and wire the feeds from the underboard stuff but this method means less wire under the board and more simplicity for stupid people like me.