Purpose:Illustrate a combination of simultaneous orthogonal oscillations.
Description:Stroke the notches with the V-shaped section on the stroker stick. The end pin will execute circular or elliptical motion, causing the propeller to rotate. Rubbing either your thumb or your forefinger on the stick (by sliding your hand slightly back and forth) reverses the phase of the horizontal oscillation with respect to the vertical oscillation of the pin, causing the rotation of the pin and the propeller to reverse.
This was once a popular toy, and has been used in "demonstrations" of psychokinetic abilities. This demonstration can be used to discuss how unexpected behaviour or difficult-to-observe variables can be used to confuse or mislead observers.