Heat in conventional pots let moisture break into steam right from the start. This steam that quickly builds up inside needs an escape or the pot could burst because the entire pot and lid get very hot and cannot condense it.
With pure clay, steam builds up only when the food is almost done cooking. And the inside of the lid stays cool enough to melt steam. With the unique lid design it gets directed right back into your food. Because of the way steam is managed right inside the pot, there is no need for a hole to let it escape.
This feature is exclusive to pure-clay cooking and is one of the reasons why food is nutritionally dense — the steam escaping the pot/pan is essentially water soluble nutrients, vitamins & minerals.
More information on MEC’s unique steam management properties are explained on this link.