This can be pretty challenging to make and there's no one-size-fits-all answer, so I recommend not relying on the text and checking out the video link below.
The idea is that it alternates where it sends the marble, hence the name. The next marble then gets sent to the other side, tipping the alternator back to it's original state. The way it works is that it sends a marble to one direction, and by doing that tips to the other side.
