Say you don't know the rotation speed of a certain motor. First, place a mark on the motor shaft with a marker. When you vary the strobe frequency of the stroboscope gradually from high to low, the mark on the motor shaft will appear to be stationary at a certain flashing frequency. When you see just one mark on the motor shaft, the corresponding flashing frequency is the rotation speed of the motor.
The following gives a detailed explanation of the relationship between the appearance of the mark and rotation speed.
1  Draw a single line on the rotary object with a marker. The diagram to the left shows what the mark looks like when the object is stationary. 

2  There is no light irradiated by the stroboscope. The object is rotating at a speed of 3,000 r/min, and the line cannot be seen by the naked eye.  
3  If we gradually reduce the strobe frequency from 16,000 r/min, we will see five lines at 15,000 r/min. This means the stroboscope is emitting light five times for each rotation of the object.  
4  When we reduce the strobe frequency to 12,000 r/min, we see four lines (12,000 ÷ 3,000 = 4).  
5  When the strobe frequency is 9,000 r/min, we see three lines (9,000 ÷ 3,000 = 3).  
6  When the strobe frequency is 6,000 r/min, we see two lines (6,000 ÷ 3,000 = 2). This means the stroboscope is emitting light for each 180 degrees of rotation. In other words, it is flashing twice as fast as the object is rotating (6,000 ÷ 3,000 = 2).  
7  If we reduce the strobe frequency to 3,000 r/min, we see just one line (3000 ÷ 3000 = 1). This flashing frequency indicates the rotation speed of the object. The line should appear sharpest at this frequency.  
8  If we reduce the strobe frequency to a value equal to 3,000 ÷ N (N = 2, 3, 4...), we see just one line. If the strobe frequency is a fraction of a whole number, we see just one line. 

9  If the line you see appears to move in the direction of rotation, the strobe frequency is slower than the rotation speed of the object. In this case, increasing the strobe frequency slightly will make the line look like it’s standing still.  
10  If the line you see appears to move in the direction opposite the direction of rotation, the strobe frequency is faster than the rotation speed. In this case, reducing the strobe frequency slightly will make the line look like it’s standing still. 