The DIAC is a bidirectional trigger diode which is designed specifically to trigger a triacs or SCRs.

Diacs do not conduct (except for a small leakage current) until the breakover (reverse breakdown/avalanche) voltage is reached.

At that point the diac goes into avalanche conduction and the voltage drop across the diac snaps back. (about 5 volts).
 
This creates a breakover current sufficient to trigger a TRIAC or SCR.

   Most diacs have symmetric switching voltages. Typical diacs have a power dissipations ranging from only  1/2 to 1 watt.