I think you are a bit confused with the terminology.
As you move from ATDC (After top dead center) to BTDC, you are advancing the timing.
Big Pig added a LOT of advance from where he was... like 18 degrees...

he was quite retarded...er...that is.... his timing was retarded, or slow....
He advanced the timing until he was 8 degrees before top dead center, then advanced it 2 more degrees..
SO, to literally answer your question, retarding it 2 degrees would bring it back to stock timing. ADVANCING it another 2 degrees might or might not cause detonation... if it isn't detonating, or running hot, you would be OK with it... but it might not be optimal for his fuel curve.. or it might give it a bit more... trial and error, with caution is indicated.
I have my timing advanced on my ATV, I had to use an offset flywheel key... as there is no distributor to tweak... I run almost 4 degrees advance.. no bad manners since doing it.
I haven't checked my timing on my kick... it runs too good to be far off..
I wouldn't run it TOO advanced...
OK, guys, through experience, how much advance can these zuki engines tolerate, and on what octane fuel??