A stroboscopic tachometer works by emitting a regular stream of very bright, very brief flashes of light. Each flash "freezes" the scene that it illuminates (like the strobe in a disco). If we shine the strobe on a rotating object and flash it at exactly the same rate that the object is rotating, the object will appear to stand still, since it will be frozen at exactly the same point in each revolution. If the strobe is slightly slower than the rate of rotation, the image will appear to rotate slowly clockwise (assuming the object itself is rotating clockwise). Likewise, if the strobe is slightly faster, the object will apear to rotate counterclockwise. This is the same effect that causes the wagonwheels in movie westerns to appear to be turning backwards. This is an example of the sampling artifact called "aliasing".
Two other alising effects require consideration, as they can lead to incorrect results. If the strobe is flashing at twice the rate of rotation, the object will be frozen at two diametrically opposite points in its revolution. If it has two-fold (or any even) symmetry, this stationary image will be indistinguishable from one stopped once per rotation. Therefore, in order to use a stroboscope, the object must be visually asymmetric. Typically a dot or other mark is placed on the object to identify a unique orientation. Similarly, if the strobe is flashing at one half the rate of rotation, the object will be illuminated once every other revoulution and will appear to be stationary.
Therefore, to assure getting a correct reading when using a stroboscope, you should start with a flashing rate known to be higher that the rate of rotation of the object. Reduce the speed of the strobe, noting the progression of decreasing numbers of frozen dots, until finally a single frozen image appears. This will be the correct speed of rotation.