Clustering, grouping mini sets together in order to keep a total rep count consistent throughout the cycle. It's simply a way of keeping TUT consistent throughout the cycle. No biggie, and even though some of us use it, it doesn't mean you'll get anymore out of it than just going with the prescribed way.
When would you use it? I use it throughout my entire cycle, but some use it only in the heavier weights.
There are a number of threads that expain it in detail, do a search on "cluster or clustering".
