If a creature with this special attack begins its turn with an opponent held in its mouth, it can attempt a new grapple check (as though attempting to pin the opponent). If it succeeds, it swallows its prey, and the opponent takes bite damage. Unless otherwise noted, the opponent can be up to one size category smaller than the swallowing creature.
Being swallowed has various consequences, depending on the creature doing the swallowing. A swallowed creature is considered to be grappled, while the creature that did the swallowing is not. A swallowed creature can try to cut its way free with any light slashing or piercing weapon (the amount of cutting damage required to get free is noted in the creature description), or it can just try to escape the grapple. The Armor Class of the interior of a creature that swallows whole is normally 10 + 1/2 its natural armor bonus, with no modifiers for size or Dexterity. If the swallowed creature escapes the grapple, success puts it back in the attacker's mouth, where it may be bitten or swallowed again. This is usually an extraordinary ability.