The way that this parameter works is already fully explained in the Ixora collected Q&A section on Data Blocks and Freelists. However, as a strategy for avoiding buffer busy waits on the freelist header block(s) during massive inserts, it is a little dated. Sure it helps a little, but with Oracle8 a much more effective strategy is to merely partition the target table.

I would like to know more about the hidden paramater _bump_highwater_mark_count. I know that it's somewhat related to freelists, but what is its effectiveness if you have massive inserts.