Ball Bearing Lock
From Spyderco glossary A compressive lock wedging a stainless steel ball bearing between a fixed anvil and the blade tang. The ball is also utilized to detent the blade into the closed position.
Which means a ball bearing forces the blade to stay open for as long as it's in place between the anvil in the knife and the tang of the blade. The force excerted on the blades "second anvil" if one might call it that, by the ball is keeping the blade in the locked position as well.