First question is whether or not you want to do ducking with stereo tracks being ducked. If so, you will have to likely implement this in a buss setup in PT/RB, though I'm not sure how it will work since PT and RB aren't really modular hosts - meaning that the signal chains are modeled after what is possible in a somewhat simple mixing board, without allowing a 'patch cord' kind of connection between tracks and so on.

In order to get it to work, you have to figure out how to route both the signal you want to compress or duck, and the signal that you will use as a control. Sometimes the controlling signal is called the 'key'. In your nano compressor, it's the side-chain input.

You need the ability to identify which signals are which in the plugin; according to how PT or RB refer to them.

Probably the most easy to understand setup for this that you've all heard, is to hear bumper music in a radio or television program get reduced in volume as an announcer's voice comes into play. The music is getting ducked while the announcer talks, then slowly (hopefully) comes back up after the announcer is done speaking.

With modular host DAW software, it's pretty easy to do this - you just connect the signals with patch cords to where they belong on the plugins, and then you can begin playing with the settings.

For ducking vocals for radio type use, you want a somewhat slow attack and release on the compression and volume reduction, slower for the release than the attack. If you make either of these too short, then an unnatural transition happens.

The other probably most common use of sidechaining that you DON'T realize you are hearing is when the kick drum sound is used to duck a bass line. The kick is the key signal, and the bass line is what gets ducked. This keeps those low frequencies present in both signals from destructive interference. In this case, the attack and release times are typically much faster. While the kick is present, the volume of the bass will be quickly turned down. This allows the combined effect of both to provide the punch without the phase competition between lows in both signals. Done well, you don't notice the effect except for the clarity it provides in a song's bottom end.

The last use of sidechain that you've heard if you listen to electronic and trance music is that bog-standard pumping supersawtooth sound. There's about 4 or 5 standard elements in trance music; this is one of them. The others are kicks on the 1/4 notes, a pretty narrow BPM range, stutter effects, snare rolls that are programmed in progression from 1/8th to 1/16th to 32nd to 64th notes over a measure, etc. In this case the effect is absolutely intentional that you hear the volume swelling up and down of the supersaw synth sound, almost as if someone is playing the synth with repetitive on the 'ands' of the beats. Not the case usually. The keying is happening with the kick plugged into the sidechain, and the synth is getting compressed/controlled in a severe manner.

Ask yourself if any of these are what you want to accomplish. Chances are 'no' in a typical song for vocals. A virtual hand on the volume fader is likely to yield better results, which my understanding is possible after-the-fact with volume envelope editing in PT/RB.

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