The split polygon tool is manual in the sense that you have to define how you want the face(s) split. The split must start and stop on edge.

With the split polygon tool selected, click on an edge to place the intial anchor point for the split (you can also drag it along an edge, it will be placed when you release the mouse button). This also creates a new vertex. Click on a second edge to add another anchor point, you see a new edge created between the two new points. When you added all the new edges/points you want press the enter key (or "Y" if you want keep using the split poly tool in a different area).

The other tool the tutorial may be referening is the Cut Faces Tool. This is akin to taking a cleaver to you model as it will create a new "ring" of edges/points all the around the model, in straight line determined by the the angle you release the tool at.