Which power device must be put in compression for out of circuit testing.
I don't understand what sort of application you have in mind here. Could you explain in a little more detail?
Anyhow..... A triac is a 3-terminal device which behaves like a bi-directional thyristor. The thyristor is triggered into conduction when a pulse is applied to its gate terminal and will continue to conduct until the main current falls to zero, at which point it will not conduct again until re-triggered.
The triac works in a similar way, but allows current to flow in both directions, and it is therefore commonly used in AC power applications (where it will shut off at each zero-crossing point of the waveform until re-triggered on the next half-cycle).
The diac is a 2-terminal device which is basically a pair of parallel, back-toback diodes. The diac is frequently used in combination with a triac in AC power controllers. The construction of the triac means that it is generally more sensitive to trigger pulses of one polarity than the other. By feeding the trigger signal through a diac first, the response of the triac can be more closely matched between positive and negative half cycles, resulting in more symmetrical waveform on the output.