LUCENE-2293 for motivation and more details.
I'm copying here Mike's summary he posted on 2293:
Change the approach for how we buffer in RAM to a more isolated
approach, whereby IW has N fully independent RAM segments
in-process and when a doc needs to be indexed it's added to one of
them. Each segment would also write its own doc stores and
"normal" segment merging (not the inefficient merge we now do on
flush) would merge them. This should be a good simplification in
the chain (eg maybe we can remove the *PerThread classes). The
segments can flush independently, letting us make much better
concurrent use of IO & CPU.