Why is it that the lid of the dish does not suck in while cooking and that it lets the steam out?

During the cooking, in Philipiak dishes emerges so-called water

ring made of steam, which obturates the lid of the dish. However, if there is too much steam from cooking, the lid will let the overflow out, realigning the pressure in the dish. If this is the case, it can mean that there is too much water in the dish, or too much meal is cooked. The dish should be filled in up to maximum 2/3 of its volume. If there is some steam going out of the dish, it can also mean that the temperature in the dish is too high, and the heat has to be reduced or turned off.