They don't rely on spring tension alone to hold the cleat in like other pedals do. The harder you try to pull out of an eggbeater, the tighter they get. Think of a scissors - if you pull on the handles, the blades close. If you're skidding hard enough to pull out of any clipless pedal, you're going to need knee surgery by the time you're 35.

I like eggbeaters for city riding because they're so dang easy to clip into.