Thank you all,That was an excellent explaination, I have got it clearly now.
In this section, jmodic say that TRUNCATE is so simple that actually data remains in the blocks while only the HWM will be reset to the first block of the first extents.
Before truncating, IF i know the HWM value , is it possible to , Kind of "ROLLBACK" by updating the HWM value back in the dict tables(Say as a SYS user).


On the other hand, when using TRUNCATE system has to perform one thing only: reset the high watter mark (HWM) to the first block of the first extent. This is a simple and extremely fast operation (if compared to all the operations described above), performed only on a couple of dictionary tables. Nothing more. So as a result, the rows actually remain written in the blocks, but anything above the HWM is "nonexistant"/"unreadable" for the system, so the database can't read those rows any more. **/