WHAT IT DOES
The tachymetre scale can be used to compute many things, but it's primary purpose is to compute the speed after noting how long it takes to travel a fixed distance (e.g. one mile or kilometer).

 

HOW TO USE IT
Suppose you wanted to measure the average speed that a racecar was traveling. After starting the chronograph function when the car passes the starting line, and stopping it after the car travels exactly one mile, you note that the chronograph second sweep hand is pointing at the 4 o'clock position (i.e. 20 seconds have elapsed). Looking at the Tachymetre scale reveals the chronograph second sweep hand pointing at 180. This means that the average speed of the car would be 180 MPH.