Solr 4.4 API / documentation

Question

I'm trying to figure out what org.apache.solr.handler.dataimport.DocBuilder is doing.
More specifically:

• In which states of the indexing life-cycle

• and in which situations it calls which callback-methods

  of the configured EntityProcessor.

(The DocBuilder class is not so easy one on the eyes)

Basically what I'm asking for is either:
a) URL for GOOD documentation about DocBuilder / EntityProcessor (Some sort of Flowchart or Sequence diagram would be terrific) or

b) Some kind soul explaining to me what's stated above