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.

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