Strobe lights can make moving objects appear to slow down, speed up, or even stop. The diagram below shows the motion of an object (from left to right) under a strobe light. If only the bolded images are lit under the strobe light, then only certain positions, or frames, of the object can be seen. These frames make up part of the objects actual movement, just not the entire thing.
The next diagram shows the clockwise movement of a knob. Only the bolded images are lit under the strobe light. So, only one of the objects positions is seen (the 12:00 position), thus making the object appear to be still. This only works if the flash rate and the objects speed are equal.