The radiator drain petcock is a plastic screw valve. It is located at the lowest corner of the radiator. Opening it lets you drain the coolant cleanly without removing the lower radiator hose. It has a small rubber O-ring to prevent leaks.
Plastic petcocks get brittle from age and heat. They snap easily when you try to open them after years of disuse. Always keep a spare plug on hand when flushing your cooling system.