How does the landing gear downlock mechanism work? Especially when it's operated without hydraulic pressure or other energy sources.
How can it keep the landing gear down and locked, during a gravity extension?
RE when it's operated without hydraulic pressure or other energy sources
Hydraulic pressure is used to release the spring-loaded lock for retraction, and that's why it's called an unlock actuator.
In a manual extension, the uplock is manually released (exact mechanism varies), then gravity takes over, and once in the down position, the springs work as they would in a normal extension, moving the lock stay to its unfolded position locking the landing gear.
Short answer just in case details in the other answers leave anybody scratching their head trying to relate:
Think of a folding table leg with its 45 degree angled brace, and the little sliding metal bit that covers its hinge. That’s the same basic principle used in most retractable landing gear designs.
The main side brace of the gear leg is itself braced to hold it straight by the smaller brace in the middle that runs back to the main gear leg. The smaller lateral brace has an overcenter position that locks it against folding up when compressed, same as your leg when you stand up and lock your knees all the way back.
When overcentered locked, the strut has a very shallow V shape and compression force just wants to push the strut harder into its overcenter lock configuration. There may also be a locking pin somewhere at the point where it folds, to fully lock the strut like that as a final security for the downlock (for example to prevent tension forces from wanting to pull it out of overcenter).
There is a small actuator that drives the brace into and out of that overcenter lock position for normal extension/retraction, but when the gear is being lowered by gravity, that actuator will be idled (free to move), and there are strong springs that pull that small middle brace strut into the overcenter lock position. It's as if someone tied a rope to your knee, and when you stood up (or if you were pulled up because your legs no longer worked), they pulled back on it from behind you to force your knee joint to go into the overcenter position where your weight can't make it buckle, and you're able to stand even though your leg muscles don't work.