In this circuit, Q1 is a simple amplifier that has its gain controlled by a variable emitter resistance supplied by FET Q2. In the up position of S1, C3 discharges through R5 and the gain of Q1 decreases because Q2 is driven toward cut-off. In the down position, Q2 conducts more, depending on the setting of R6,
which causes a gain increase. By varying R5 or C3, various fade rates can be obtained.