The radio signal is amplified by the valve and is then detected
by the crystal detector (a valve being used in later receivers).
The audio signal voltage is then stepped up by the transformer
and passed back to the grid of the valve through the A.T.I.
(Aerial tuning inductance). The small condenser is required to block the audio from being shorted out and also to bypass the RF signal to the valve filament and so complete the A.T.I. circuit.
Reflex operation has also been used in some later superhet receivers where the IF amplifier valve also acted as an audio pre amplifier too.
During World War 2 the very common lightweight Army portable set, WS38 used this reflexing.