In simple terms:
It makes things very, very
cold.
In slightly more complex terms:
It makes things so cold, they practically stop
moving.
In scientific terms:
When a
substance is exposed to a magnetic
field, there is a larger amount of electrons with a -1/2 spin than there are with a +1/2
spin. This is a loss of '
randomness' (the lowering of the
entropy) at a constant temperature. So, if you cool something with 1k liquid
helium, then expose it to a strong magnetic field, an
isothermal process, it causes heat to leave the sample towards the cold reservoir (the helium). The
electrons adopt a lower
energy state and the entropy of the substance is reduced. The helium is pumped away and the
magnetic field is removed. These changes are
adiabatic and
reversible, and so leaves the sample with only a lower entropy, thus lowering the temperature.
A slight variation in this method,
adiabatic nuclear demagnetization, has been used to cool a sample to 20 nK.