Modern chronys have two light sensors on them at a fixed distance apart. They are activated by the shadow of the bullet crossing them. They have a very accurate clock built into them and the bullet starts the clock when it passes over the first sensor. When it passes over the second light sensor the clock stops. Since the distance between the two sensors is fixed it's a simple matter for the processor to calculate bullet speed by dividing the time for the bullet to travel between the two sensors into the distance between the sensors.