How can this slowly moving ball bearing translate into enough power to launch the can from the table?
Neodymium magnets, that's how. The bearing is rolling towards a very powerful magnet; as it gets close, it's accelerated powerfully, and the momentum is transferred, Newton's cradle style, to the ball at the other end of the cluster (which isn't held as strongly by the magnetic force, being further from the magnet). Repeat that acceleration process a few times, and you get an enormously powerful multiplication of force. This video explains the process in more detail:
Unfortunately, you can't repeat the magnet clusters indefinitely to get the ball bearing accelerated to, say, 90% of the speed of light. After 5 or so clusters, the velocity of the ball causes the magnet to shatter (neodymium is quite brittle), so that's one reason why we don't have neodymium-magnet rifles on the battlefield. Another reason is that the energy of the projectile doesn't come for free; it's drawn from the bearing at each stage falling down a magnetic well. You'll have to pay that energy back each time you "reload" the gun -- it takes quite a lot of effort to remove the bearing from the magnet while it's stuck there.