Ducking is a popular technique used in EDM music to apply percussive processing to pads, leads, and bass lines using side-chains on compressors. This technique is also used as a method for getting bass and drum passages to subtly fit together in mixes. The most notable sound for ducking is the sustained synth that sounds like it is pulsing to a beat. That pulse is created by another instrument. In order to successfully apply ducking to your track you must have the following:
Compressor inserted on a pad or lead track with side-chain capabilities
Percussive source to key the side-chain
Check out this video explaining how it is done, then try it yourself.