The heroes have struggled long and hard, but they have finally beaten everything that the Big Bad can throw at them. They have clearly defeated him. The villain, rather than trying to escape or try and take the heroes with him, graciously acknowledges their victory and yields, surrendering himself to their justice, resulting in a Twist Ending (or really, an Untwist Ending). Can happen more often in series where there is a Cardboard Prison involved. A villain who happens to Know When to Fold'Em just may do this. Can also happen when he chooses to Face Death with Dignity.