(From the Marbles: The Brain Store site) Pull back the SNAPping bar and place all of the discs on the other side. Release the tension on the bar and let it slide into place. Then, take turns removing one disc at a time.
If you remove a disc without the bar snapping forward you can keep the disc and points are added to your score. However, remove a disc and the bar lunges forward, discard the disc and gain no points.