I posted about this a while ago, but was reminded about the Tourbillon by this post on Reveries. Being somewhat of a watch collector myself, I am fascinated by what is considered the ultimate test of a high-end watch manufacturer. Unfortunately I cannot say that I have a watch with a tourbillon yet in my collection, but I am always on the lookout.
Essentially the tourbillon counters the effects of gravity that may cause problems with the precision of an automatic watch. If you have never seen a tourbillon (which means “whirlwind”), it is a mesmerizing thing to look at.