The thyristor module turns on by applying a pulse current to a gate current.
However, because it is a latch type semiconductor, the thyristor only turns on when a sustained current (holding current) reaches a set level.
To turn off in a DC circuit, the gate current must be reduced below the holding current or reverse voltage has to be applied using a commutation circuit. In an AC circuit, the pulse current has to be applied to the gate current whenever it is positive, because the gate current falls below the holding current and the thyristor turns off whenever the current alternates.