Is anyone able to explain this phenomenon?

[Daz02]

I inserted a key into one of my locks (the key did not belong to the lock) and of course when I inserted it all the way in it would not open because the key does not belong to the door, but to my surprise, when I was slowly pulling the key out I was also twisting it and it unlocked the lock. It turns out that you can unlock it with that key if you get the key in the right spot even though that key does not belong to it. What is the explanation behind this?

TL;DR: I inserted a key that does not belong to a the lock and for some odd reason it opens it when the key is in a certain spot inside the cylinder

[LocksportSouth]

Sounds almost like using a jiggler key. I picked up a couple of these for my cruciform lock and they work a treat - by having a (basically) bump key for the keyway you can zip it in an out and the kinetic motion sets the pins. I guess the other option is that the cylinder is not fully pinned and the bitting in your key is identical / close to the real biting positions for that lock when pulled out by one space .

I could be totally wrong, but that's my guess.

[kwoswalt99-]

To know why it works, you have to know how a lock works. It just happens that the wrong key lines the pins up at the shearline, at that particular spot on the key.
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An old friend of mine, who's a locksmith, showed me this when I was little. He frequently opens locks like this. Just takes a little playing around with "lower" cut keys.
Hello, and welcome to the forum!

Edit: Locksportsouth: you are correct. Most often you have to try different positions between where the key naturally sits, and jiggling it helps as well.

[cledry]

Usually happens on locks with sloppy tolerances and/or more wear. Possibly your lock has master pins in it too.

[Mr_Bones]

This seems similar to the technique of overlifting a wafer lock, where you insert a blank key and slowly pull it out while applying tension.

[billdeserthills]

Key picking at it's finest