Complication guide
Tourbillon: how the complication works
A tourbillon mounts the escapement and balance in a rotating cage, turning them to average out the effect of gravity on the watch's rate.
A pocket watch sitting in a waistcoat spends its life in a few fixed positions, and gravity pulls unevenly on the balance in each, changing the rate. Breguet's answer was to put the whole escapement into a cage that rotates, so that over each turn gravity acts on it from every direction and its influence averages out.
On the wrist, where the watch is constantly changing position anyway, the practical benefit is small. The tourbillon survives as one of the most admired feats of watchmaking, a moving sculpture of tiny parts that a maker builds to show what it can do rather than to keep better time.