Here is an animation showing how to unlock a lock with (or without, for illustration purpose only) a key! Mordent locks have more complex structures, but they essentially follow the same principle. The complex design is to make sure that simple tools like that shown in the animation won’t be able to unlock it.
Junling got his PhD in Materials Engineering from University of Maryland, College Park. He is currently a Professor at Nanyang Technological University, Singapore, teaching undergraduate courses related to Quantum Mechanics.