A plastic channel supports the row of 4 ball bearings, and a pair of neodymium magnets at the end of the row hold all the ball bearings together magnetically.

Very slowly, you roll the loose ball bearing towards the magnets. As it approaches the magnets, it and the magnets are pulled together sharply, since these are very strong magnets. And as a result, rather like that Newton's Cradle executive toy we used to see a few years ago, the ball bearing at the other end shoots off at a surprisingly high velocity.

http://www.grand-illusions.com/acatalog/Gaussian_Gun.html