They've become common in NFL multi-year contracts. If they're not excluded in the CBA I expect they'll make their way to the NHL rather quickly. They're used by teams and players, to get the best possible outcome. Here's one that favors the player from the Atlanta Falcon's website. http://www.atlantafalcons.com/team/article.jsp?id=2685 Obviously this is for a backup or developing player that wants an out if he becomes a starter so he can get a higher contract. Teams could also do that with stars, making years voidable if players don't hit certain performance incentives. In the NHL, it would be attractive for players to have voidable years based on league revenues. For example a guy could sign a 5 year deal, but it's voidable after 3 if the maximum cap salary allowed increases more than 20%.