Why do spool pins make locks harder to pick than regular pins? Because the narrow waist of the pin will get stuck at the shear line binding the lock so it can’t turn any further.
The good news is this only happens once the other pins in the lock are already lifted to the shear line. Assuming a lock has only one spool pin,
you’re almost there once the spool pin gets bound. You can feel this happening because the lock plug will rotate slightly when it happens. Unlike the barely noticeable rotation and click when a regular pin reaches the shear line – the binding of a spool pin is much more obvious. The rotation is enough that you can see the plug has turned a little – but just a little.
So what do you do when a spool pin has bound and the lock rotates that tiny bit before becoming helplessly stuck? Continue to lift the spool pin slowly while letting the lock rotate backward as the pin is lifted. If you’re slow and careful enough (and lucky enough), the spool pin will reach the shear line without the other pins dropping back to the locked condition.
If you’re not lucky enough and some of the other pins drop, that’s okay. Now that the spool pin is at the correct height, you can continue to single-pin-pick the lock normally.
Spool pins are tough to overcome but they are a common feature of many different locks today and the hobby lock picker will come across them fairly regularly in the real world.