Andy,
You can find (out there on the web) a very detailed explanation of the workings of the the gears and governors and tapered post, etc. within the speedometer, but I think it boils-down to the chronometric needle making slight jumps up and down as it catches up with the speed sample measured by the mechanism. This may not be the best way to say it, or be spot on regarding the workings of the clock, but this is how I basically understand it. I will be quite happy to read other explanations myself.
Richard L.