It also has a knob on the side which you have to turn 10 times, every so often, and if you forget to do it, you enter a condition variously known as "lost" or (in the authorised language of ex-RAF navigators working at the CAA) "uncertain of position".
There are other ways to enter this condition; for example if you spend too much time looking out of the window then you might forget to notice that the stopwatch has passed the time of the current leg.
Another way to forget to monitor the stopwatch is to be enjoying the flight too much. God forbid! Ze British are not supposed to ENJOY somefing
Then, if you look down and below you see two long parallel runways, with a load of 747s waiting at the hold, and with the sky above full of more 747s flying holding perfect 1-minute holding patterns (gosh
what a lot of stopwatches there must be flying around up there...), you can expect
a) visit from the CAA, and
b) prominent mention in the aviation magazines
With b), the production of the stopwatch will get you an automatic exemption from unfavourable coverage (it's not necessary for it to be functioning; the
possession of a stopwatch implies due diligence and "good airmanship"). However, if a GPS is found among your possessions then you better wear a hood next time you go flying.
With a), they will prosecute just the same.