The two things will happen when the tablespace in backup mode

1.) The datafile header associated with the tablespace in the backup up mode will be in frozen state.So it stops updating the datafile header, but it continually writes into the datafile.

2.) More redos are generated.This is because oracle will write all the changed blocks into the redo log.Normally only the changes are logged into the redo logs.

regards,

Noyal