SWP stands for: Swiss Water Process. A non-solvent method for decaffeinating unroasted coffee beans developed in Switzerland.
Coffee beans are soaked in hot water so that the caffeine is extracted from the bean and into the water. The resulting liquid is then processed through activated charcoal to remove the caffeine. The coffee beans are returned to reabsorb the remaining liquid and, presumably, the flavor.
The Swiss Water Process extracts between 94 to 96% of the caffeine content of a coffee bean!