If the Stormlight in a gemstone is withdrawn quickly enough, a nearby spren can be sucked into the gemstone. This is caused by a similar effect to a pressure differential, created by the sudden withdrawal of Stormlight, though the science of the two phenomena are not identical. You will be left with a captured spren, to be manipulated as you see fit. —Lecture on fabrial mechanics presented by Navani Kholin to the coalition of monarchs, Urithiru, Jesevan, 1175
