Block B is a phase shift circuit which enables the phase of the pulses to be shifted with respect the ac supply.

These pulses are applied to the gate.

In waveforms A and B, the gate pulses coincide with the start of the positive half cycles of the mains supply.
They turn the thyristor on, and current flows for the complete half cycle.
There is no current flow during negative half cycles.
The current flow is shown at C.

Waveform D shows the gate pulses shifted and occuring half way through the positive half cycles of the ac supply.
The thyristor starts to conduct at this time.
Current flow is shown at E.

F and G show the gate pulses occuring near the end of the positive half cycles, and current flowing for a very short time.

The circuit acts as a lamp dimmer.
However, current only flows during half of the ac cycle, and full brightness cannot be achieved.

The high power thyristor circuits can be isolated from the control circuits by means of transformers or opto isolators.