Your body heat and surrounding temperature will slowly make the water inside the vest vaporize. This causes the surface of your skin to cool down before you start to sweat. Depending on the temperature and humidity, a 600ml fill will last up to 3 days. The filling cap has evolved into a special waterproof zipper construction at the back.
The heat from body and surrounding air make the water inside the vest vaporise. It causes the surface and your skin to cool down in a dry way before you become sweaty.