The photo is of the Model 1876 Winchester which was the last and largest of the toggle joint design leverguns.. The design is the same as the Model 1873.
The rear of the toggle joint is anchored by a pin through a shoulder milled into the frame of the rifle. The forward link of the joint is attached by a similar pin to the rear of the breech block. Throwing the lever down (forward), which is attached to the middle of the toggle by a pin, collapses the toggle and slides the breech block out of battery to extract the fired case. At the end of the forward motion of the lever the follower raises a new round into position. Rearward movement of the lever slides the breech block forward which chambers the round. Full movement of the lever to the rear, cams the toggle joint into a "locked" position.
It is not a strong method of locking a breech block, though it works fine with black powder pressures. Shooters should be aware and load accordingly.