Hey loopdon. I developed this technique myself, though I'm sure there are people out there that use a similar trick. It gives me ALLOT more flexibility than just setting linking the gain to a peak controller and inverting the mapping. The nice thing with this setup is that you get to specify exactly how much you want to compress/expand it, as well as overalll gain control. Another really big bonus is that you get to see exactly how the function is operating by clicking on the "meter" tab. That's quite useful for fine tuning your peak controller's tension feature. It's a far better solution in general...

Anyway, down to the maths

The formula (a*(2*b-1))+c has three "active ingredients" a, 2*b-1 and c

the fuction f(b)= 2*b-1 translates and linearly scales the values of b from the interval [0,1] to the interval [-1,1]. More specifically, at exactly half way (b = 0.5) it will evaluate to 0, i.e. 2*0.5-1 = 0. Likewise, at b=0 f(b) evaluates to -1 and at b = 1 it evaluates to 1.

In this situation b is used to control the "polarity" and intensity of the side chaining action. By polarity I mean wether it's going to increase the gain when your peak controller is triggered or wether it's going to reduce it. For instance, by setting b = 0.45 you get a very minor dip in the output of the overall function, but by setting b = 0.1 the effect is markedly bigger (so you get chunkier compression). If on the other hand you set it to a value > 0.5 it'll actually boost it. Play around with it and you'll see what I mean.

the parameter a is used to directly scale (2*b-1), and is linked to the peak controller.

c is the value that reflects the function output when your peak controller is outputting 0 i.e. the controlelr value when there's no kick drum or whatever you're using as your key. c could therefor be called your base level. I like to set knob to 79%, which is the value which fruity's mixer gains defaut to just to keep things nice and uniform.

Some good settings using my technique for some daft punk style side chaining are as follows:

a is linked to peak controller, b = 20% (0.2), c = 79% (0.79)

Hope that cleared things up.

By the way, have a try at this controlling other things in FL Studio... some suggestions to get you started are: reverb send levels, synced high pass filter sweeps on basslines instead of gain reduction or even plugin parameters (bit reduction and distortion works nicely here) As you can see this is a pretty potent technique, and the primary reason I don't use another sequencer for writing techno. Internal controllers rock!!!