My home inspection of Speedmaster heads

I'm not the one to explain this
But i will try.
When the spark plug fires it takes a moment before the flame is big enough to expand and push the piston down.
we advance the timing until we get the best part of the flame/heat/expansion just after tdc.

YR is implying that the short spark plug caused the fuel expansion to slow causing the extra timing, required, for for optimal burn at the right time.
Please correct me if i didn't get that right............