It's to do with the sidechain. The sidechain puts a control voltage into the audio circuit. The audio circuit is balanced to cancel the sidechain signal out. The balancing circuit is just to fine tune the cancellation because both sides of the circuit are unlikely to be perfectly matched. The reality is the gain curves under compression will not be exactly the same, so although the balancing circuit works quite well it is still a bit of a compromise. You will only notice the thumping from the sidechain when the box is actually compressing & if the 2 halves are out of balance. One way to do this is to connect a signal generator between the earth & the point that the sidechain connects to (I think on the low side of the input pot, sorry I don't have diagram) then adjust the trim so that you can hear the smallest amount of the oscillator. It is worth being aware that at different levels of compression the null point is different unless the gain curves of both halves are perfectly matched.
