When the trIGger current ig is applied to the gate, the thyristor will be triggered and turned on at a lower anode voltage. The larger the gate current ig, the lower the corresponding steering voltage. See the abscissa corresponding to IG 1 and IG2. Do you know what the conduction condition of thyristor is? Adding trIGger voltage to the gate actually provides a certain trigger current ig, which is included in the specification of thyristor. The knowledge about thyristor is not here, but the key is the on-off condition of thyristor and the selection of rated voltage and current. Note that thyristor is a semi-controlled device.