How does a cryo-pump work?

Previously, I have discussed the cryotorr 8, but how does it work?
A small presentation is shown at the helix website. The cryo-pump uses helium as cooling agent. It cryo-condenses water, nitrogen and argon by frosting them on the condensation array. By using activated charcoal, it cryo-adsorbs hydrogen, helium and neon. Water is cryo-condensed on the first stage array, at ~65K. Nitrogen and argon are cryo-condensed on the second stage at ~12K. Hydrogen, helium and neon are cryo-adsorbed also on the second stage.
 By cryo-pumping, we are able to lower the pressure to ~3e-11 Torr from 8e-11 Torr.