Defying gravity

Each of these rings will slide easily up and down the pillar in the middle, so why are the red and green rings not dropping down the pillar to rest on the other rings?

I am sure that some of you will recognise this little demonstration of magnets repelling each other. Each ring is a magnet: if you turn them up the "wrong" way with like poles facing, the repulsion stops the upper rings from dropping down the pillar, and they just hover there.