An internal spring locking mechanism keeps them holding on tight until you separate the disks releasing the lock from the post on the pin. The best part is no tools are needed to lock or unlock these backs!
These can be a bit tricky to release if you are new to them, but with a little practice you will be a pro. Just squeeze your fingertips together toward the end of the backer, this will slide the flat part up and release it. Do NOT try to forcibly remove them, they can damage the post if you pull hard enough.