When I first started picking, I often encountered a problem with locks that baffled me. I would lift some of the pins to shear and then encounter a pinstack that wouldn't budge. It was obviously not set or even disturbed. But When I put my hook underneath it and tried to lift, it felt like my pick would bend before I got the stack to move. So I reasoned that my tension was too great, and backed off considerably. Sometimes this worked, and other times it was not sufficient.
I dropped the picking for several years, and when I came back to it, I started taking my locks apart. That was how I discovered what was behind this "frozen pinstack" phenomenon. If the key is cut extremely high (in certain lock brands), then the bottom pin for that stack will be so small that it will allow the top pin to fall completely inside of the plug (with only the spring grazing the surface). After you break a few other pins , the ledge becomes so severe that the top pin of the stack in question can no longer enter the shell. So you have both pins stuck in the plug. And depending on the specifics of the lock, a light enough tension to get it past may just reset all of the other pins (although it's usually the solution).
In some cases where a lighter tension isn't the solution, it's solves the problem to lift that stack very slightly into the shell and then apply tension to hold the top pin there as a preliminary to picking the lock.
Cheers
(I should really replace the driver...)
