How does this circuit control motor speed?
It's a dimmer circuit, electronics-tutorials has a really great article on it: http://www.electronics-tutorials.ws/power/diac.html



"If we wish to control the mean value of the lamp current, rather than just switch it “ON” or “OFF”, we could apply a short pulse of gate current at a pre-set trigger point to allow conduction of the SCR to occur over part of the half-cycle only. Then the mean value of the lamp current would be varied by changing the delay time, T between the start of the cycle and the trigger point. This method is known commonly as “phase control”."

Looks like L1 is just there as a low pass filter, it's hard to tell without values, but probably the switching on and off of the Diac creates a sharp rising edge that would create unwanted audible noise in the circuit.