Top Down Clustering works in multiple steps. The first step is to find comparative bigger clusters. The second step is to cluster the bigger chunks into meaningful clusters. This can performance while clustering big amount of data. And, it also removes the dependency of providing input clusters/numbers to the clustering algorithm.
The "big" is a relative term, as well as the smaller "meaningful" terms. So, the control of this "bigger" and "smaller/meaningful" clusters will be controlled by the user.
Which clustering algorithm to be used in the top level and which to use in the bottom level can also be selected by the user. Initially, it can be done for only one/few clustering algorithms, and later, option can be provided to use all the algorithms ( which suits the case ).