Wave Folding is a west coast synthesis methodology where a sound source is amplified and then "folded" over the ±1 mark to fit in the normal -1 to +1 audio signal range.
Don Buchla pioneered this method in the late 1970s.
In this example, the wave can be folded 4 times. As you move your mouse from left to right, the amplitude is increased. The tops and bottoms are then inverted over and over. The result gradually turns a sine wave into a lot of smaller triangle waves that harmonize with the principle frequency. Adjust the frequency by moving your mouse up and down.
This method is effective in creating more complex sounds than the east coast method of starting with a harmonic-rich sound and filtering out the low or high ends. The result is a lot of different timbres.