In the circuit, the cathodes of the diodes are more positive than the anodes.
They are reverse biased and non conducting.
The output of the circuit is isolated from the input.

When the negative gating pulse comes along, the cathodes become more negative than the anodes.
The diodes are forward biased and conduct.
The output is connected to the input.
During the duration of the gating pulse, the input signal appears at the output. as shown by the lowest waveform.