This is a subtle trick that doesnt get used as much as it used to, but its still a great technique to know about. The idea is to recreate the sound of a tape machine or compressor dipping the volume and treble of a track when the kickdrum is heard. It was used a lot on 'Golden Era' hip-hop records, and Portishead used it a lot too.
This is the basic 'recipe' for sidechain pumping:
1- Route your kickdrum into an FX channel of its own.
2- Add a Peak Controller FX to it. make sure you turn muting off on the plug-in.
3- Add a Volume FX to every other track except the kickdrum and the main output.
4- Right-click each Volume FX and select Link To Controller.
5- Select Peak from the dropdown list.
6- Uncheck the Remove Conflicts box
7- Go back to the Peak Controller Plugin
8- Turn Base all the way up, and Volume down to around 30%.
9- Make the TNS shape point upwards.
10- Tweak the Decay parameter to taste.
You should now hear the kickdrum 'pumping' the rest of the track.
Enjoy, you Floopers.