This is made possible by supercooling. Supercooling is the process of lowering the temperature of a liquid or gas below its freezing point WITHOUT it becoming a solid. The moment you disturb the supercooled water(below 0 degrees celcius), the water will freeze instantly and will even increase in temperature to 0 degrees celcius.
Want to try this at home? Follow this link to find out how to make supercooled water at home! (If you are having problems, try an unopened bottle of distilled water. Because if the water have impurities, it will be hard to supercool the water.)