Sidechaining is not something I'd ever use at the mastering stage. For me it's a fairly early mix sort of thing and you use it after you've spotted that one of your instruments is getting in the way of another (unless you're aiming for a Daft Punk style result in which case you use it on pretty much everything).
It's just a technique which can be applied to many different FX. IMO it's usually used with a compressor to reduce volume of one instrument rather than cut frequencies. The commonest use is probably when you have kick and bass both around the same frequency and you use sidechaining to duck (reduce volume of) the bass when the kick plays so that the kick can stand out better in the mix.
OTOH perhaps you're talking about sidechaining with some other FX ?
Steve