From the web page:
This EntityProcessor imports data from different Solr instances and cores. The data is retrieved based on a specified (filter) query. This EntityProcessor is useful in cases you want to copy your Solr index and slightly want to modify the data in the target index. In some cases Solr might be the only place were all data is available. The SolrEntityProcessor can only copy fields that are stored in the source index.
Due to appear in Solr 3.6! So, grab a nightly build for now if you want to start learning this feature.