What is High Water Mark (HWM) ?

The high water mark level is just a line separate the used blocks and free blocks.

The blocks above the HWM level is free blocks, they are ready to use.

The blocks below the HWM level is used blocks, they are already used.

See the below example:

If you delete some huge records from the database, that data will delete but the blocks are not ready to used, because that blocks are still below HWM level, so delete command never reset the HWM level,

At the same time you truncate the date, then the data will delete and that used blocks will goto above the HWM level, now its ready to used. now they consider has free blocks.

I hope this enough to understand.

+---- high water mark of newly created table

|

V

+--------------------------------------------------------+

|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

      high water mark after inserting 10,000 rows

                                    |

                                    v

+--------------------------------------------------------+

|x |x |x |x |x |x |x |x |x |x |x |x |  |  |  |  |  |  |  |

|x |x |x |x |x |x |x |x |x |x |x |x |  |  |  |  |  |  |  |

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

      high water mark after inserting 10,000 rows

                                    |

                                    v

+--------------------------------------------------------+

|x |x |x |x |x |x |x |  |  |  |  |  |  |  |  |  |  |  |  |

|x |x |x |x |x |x |x |  |  |  |  |  |  |  |  |  |  |  |  |

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Hope this helps...