Lockback
At their core, lockbacks (also referred to as back locks, spine locks, or mid locks) are simply a derivative of the non-locking slipjoint. The lock bar is pinned to the scales of the blade, pivoting in the middle, and a bent spring anchored further back in handle provides upward pressure behind the pivot point, pressing the front of the lock bar downward. In the closed position, the lock bar sits on a ramp in the bottom of the tang which provides a detent for opening. >> Examples of lock back knives Use this link to see and purchase a Lockback knife.
When the blade is opened, the forward portion of the lock bar with its square protrusion sits down into a matching square cutout on the top of the blade tang, locking it into position. The cutout on the tang matching the shape of the lock bar means that the lock bar has to be lifted out of the notch to release the blade. The spring bar thus keeps the lock closed until the bar is pressed from behind the pivot point against the spring tension opening the knife.
Thanks to KnifeInformer for the info