Sidechain compression, or 'sidechaining', is the use of one audio signal to trigger the compression of another. Most commonly, this is used to create a pumping effect, and also to ensure that certain elements can be heard against another in a competing frequently range. For example, a kick is often 'side chained' to the bass instrument, so that when the kick is triggered, the volume of the bass is reduced.
There is an effect on the MPC that enables Sidechain: Mother Ducker
Firstly, on the track that you want the compression/volume ducking to occur on, load the Mother Ducker effect on the channel as an insert. You can add this from the channel mixer, or by viewing the channel strip in main mode (opened by pressing the eye icon to the left of the sequence tab).
Once loaded, you will need to add the Mother Ducker Input as an insert effect to the channel of the trigger. Open this effect, and set the channel so that both the input and actual effect share the same bus number. This pairs the trigger signal to the desired destination, where it will duck the audio.
On the Mother Ducker effect, reduce the threshold until you start to hear the volume duck when the trigger signal is received. You can also use the other settings to impact how quickly the effect is applied and then stopped (attack and release), as well as the amount of compression applied (ratio).
For more information, we also have a video tutorial available, linked below: