How does the B777 Side brace locking mechanism work?

The scissors struts on that gear physically lock in alignment instead of over centering. The inboard strut extends into the outer one a short distance, which is forked to accommodate it. The little actuator with the lever and springs is the downlock mechanism. The downlock will have some sort of pawl or plunger interface hidden inside that forked interface, roughly where the downlock safety pin is, that engages to lock the inner strut and outer strut together so they are like a one-piece strut.

The actuator pulls in to disengage the mechanical lock, and the springs provide a counter pressure against the actuator to provide a passive "engage" force the rest of the time. The springs aren't maintaining an over-center condition, only applying pressure to a mechanical lock.

In this video you can see that the downlock release actuator appears to also pull the links out of alignment once the release lever it operates is fully released, and if you watch full screen you can make out the locking mechanism as the struts fold, although the precise details of that mechanism would require access to a 777 maintenance manual.