Actually it IS the draft that clears the area above the fire, and in the heat exchanger once the inspection door has opened..
We had an AHS owner,, maybe it was an AA, that had real problems with puff backs last year,, they installed a draft inducer for use in warmer weather when thier chimney wasn't pulling a very strong draft.. it seemed to cure the puff backs
I think a light draft, just when the inspection door opens, pulls just the right amount of oxygen into the firechamber, mixes and is ignited.. If the draft is strong, the combustibles are quickly pulled out of the firechamber.. it's a tightrope walk.. just the right combination causes the puffbacks..
The 5/8" hole adds fresh air over the fire while the fan is running, diluting the gases above the fire,, this is all that is needed to make the combustionable mixture too 'lean' to ignite..