[1] This is most easily done by bringing liquid hydrogen near its boiling point and then reducing pressure using a vacuum pump.
The decrease in pressure causes the liquid hydrogen to vaporize/boil - which removes latent heat, and ultimately decreases the temperature of the liquid hydrogen.
The vacuum pump is stopped, causing an increase of pressure, the solid hydrogen formed on the surface partially melts and begins to sink.
The solid hydrogen is agitated in the liquid and the process is repeated.
[2] It is proposed as a rocket fuel in place of liquid hydrogen in order to use smaller fuel tanks and thus reduce the dry weight of the vehicle.