Evaporative cooling is the mechanism that cools a hot cup of coffee. The most energetic molecules are the ones most likely to escape as steam, and in the process lowering the average kinetic energy (and therefore the temperature) of the coffee. The same thing will happen with particles in a trap. If the trap depth is low enough, the highest energy particles will be able to escape and lower the temperature of the remaining population. We can encourage this process by intentionally lowering the trap depth to release more particles and force the temperature lower and lower.
Evaporative cooling of neutral particles in a magnetic trap was one of the key techniques that enabled the production of the first ever Bose-Einstein-Condensates. In ALPHA, we use evaporative cooling to cool antiprotons and positrons in our Penning traps. With this technique we can reach plasma temperatures below 10 K.