The Presto 6 qt. pressure cooker is a timeless piece of kitchen equipment. This is a simply constructed appliance: the simplicity is a strong point for this product.  The pressure controller is a weight that sits on a small tube on the center of the lid. As long as the opening is intact it will always keep the correct pressure. The safety plug is a small rubber insert that sits to the side. If the cooker over-pressures this safety will pop out.  When the pressure cooker is pressurized the little plug protrudes, showing there is still pressure in the pot. An interlock, a small metal plug that internal pressure pushes up, prevents the top from being removed if there is pressure in the pot.  Therefore the mechanics of this unit are drop dead reliable.