SHIMPO DT-315P - Manual (Page 6)

5. Measuring rpm is done by turning the setter dial.
Turn the EXTERNAL KNOB to adjust the flashing rate to the
rotating speed of the object. The greater the turning
angle, the faster the flashing rate.
Turn the INTERNAL KNOB to adjust the flashing rate to
the rotating speed of the object precisely until the object
appears to be standing still.
When the frozen motion is obtained, press the x2 or ÷2 button:
When the x2 button is pressed, 2 frozen figures can be seen overlapped by
180 degree phase shift.
When the ÷2 button is pressed, a freed figure can be seen.
At this moment, the actual rpm of the rotating shaft can be displayed.
The phase shifted frozen motion can also be seen when the displayed rpm is
1/2, 1/3, 1/4, etc., of the rotating shaft. When the x2 or ÷2 buttons are
used effectively, the correct rpm can be attained.
When the + button is pressed, the frozen figure will shift towards the
rotating direction.