how does ddrescue's automatic merging work?

so i've just come across this information about ddrescue:

'Automatic merging of backups: If you have two or more damaged copies of a file, cdrom, etc, and run ddrescue on all of them, one at a time, with the same output file, you will probably obtain a complete and error-free file. This is so because the probability of having damaged areas at the same places on different input files is very low. Using the logfile, only the needed blocks are read from the second and successive copies.'

having had a drive (sdg) fail on me i originally ran this command to make a disk image of the faulty drive (carried out in reverse) to a new drive (sdh):

---------------------------FIRST REVERSE PASS-----------------------------------
ddrescue -R -d /dev/sdg O_Faulty_DD.img test.logfile
-------------------------------------------------------------------------------------

now with ddrescue's automatic merging, does that mean i can now run a command that will attempt to recover the data from the beginning of the failing drive (sdg) by adding/merging whatever it can recover from a FORWARD pass command (below) to my already saved image file (O_Faulty_DD.img) without wiping out the O_Faulty_DD.img?

=================NEW FORWARD PASS============
ddrescue /dev/sdg O_Faulty_DD.img test.logfile
==========================================

this i ask as the process has already recovered almost 700GB from the faulty drive (in reverse mode) and i'd be pretty gutted if i had to start all over again simply because i issued ddrescue with an incorrect command.

any help would be much appreciated.

thanks

anybody?