The spool pin is the entry-level security pin. When lock manufacturers incorporate security pins, the spool pin is usually the pin of choice. The Spool pin is named so because it resembles an empty spool you would wind string or yarn.
Spool pins (Illustration ::0) are the most commonly found security pin due to the low manufacturing cost compared to other security pins. They add very minimal pick resistance and really only keep out novice pickers. The way these work is while pushing up on the key pin, you'll first push the top of the spool past the shear line. This will cause the core to rotate a small bit, putting it into what's called a "false set" since the smaller mid-section of the pin is caught in the shear line. This is where you think the pin was set to shear, but in fact it was not.
An alternative version of the spool pin is the Serrated Spool pin (Illustration ::1). I have only ever seen these in American locks. They combine the pick resistance of a serrated pin AND a spool pin and can be a little tricky if you haven't yet developed a feel for it.
Another alternative version of the spool pin is the Drunken Spool pin (Illustration ::2). This style of spool pin is fairly uncommon and I have only known of these to be in some Winkhaus locks and the occasional Yale style mortise.
Detecting a spool pin is rather simple. When you think you have set a pin to shear and the plug rotated a little, push up on the keypin a little more and you should feel feedback on your tension wrench. This is because the wider bottom of the spool pin is trying to fit through the partially closed shear line. This will cause the plug to want to rotate backwards a small bit.
Defeating a standard spool pin is quite easy as well. When you get the counter-rotation indicating a spool, what you want to do is very subtly let up on your tension wrench while applying a light (but constant) upward pressure on the key pin. Keep letting up on your tension wrench until the spool pin clicks past the bottom of the spool and to the actual shear line. You will get a sharp click and a rotation of the plug.
A serrated spool is really no different. First you'd treat it like you would a serrated pin, and once you get past the serration it will then act like a spool. Once you're past the spool, it will act like a serrated again; so it's really not much more difficult. Sometimes if you think you've lifted all pins to shear and the lock still doesn't open, you may have to intentionally overset one pin at a time, allowing it to fall back down each time, to test for serrations or other security features.
For practice with spool pins, you can try most any Schlage lock at any chain store, you can also try any laminated Brinks padlock. If you're into a bit of a challenge, you can try any of the Abus Titalium series padlocks. Abus LOVES to use spools, plus some (usually) tricky warding. This can sometimes pose quite a challenge to pickers.