Regarding HOt back up

What happened internally " when we put the table space in begin back up mode? 
And why more redolog is generated at that time?

You should use RMAN for making database/tablespace backups because RMAN is the 
only backup tool that "understands" fractured blocks during recovery (blocks 
that were changed while the backup was taking place).

If you are not using RMAN then you have to tell ORACLE not to write any changes 
to the datafiles while you are copying them -- you do that by putting 
tablespace in BACKUP mode (BEGIN BACKUP).  When tablespace is put in backup 
mode it forces ORACLE to STOP modifying data blocks in all datafiles of the 
tablespace.  Where do the changes are written then?  They are written into the 
REDO stream -- that's what's causing the redo logs to switch frequently.

Use RMAN instead.