Lots of ideas and opinions abound .
FWIW - sometimes an advance of up to 50 degerres is required on some 4 strokers .
The amount of advance is related to the pressures developed in the combustion chamber -and WHEN they are best ignited
No secret.
Also in some engines, the pistons spend far more time as they travel over the top- than they do as they travel thru the bottom of a stroke - (has to do with relative rod to stroke length) and these types can take advantage of INCREASES in timing.
No secret
Al;so once th e engine reaches max torque point - furthe increases in rpm DECREASE pressures in th cylinder
no secret
further increase in advance does not cause pre ignition
THAT occurs when the charge ignites too early along the piston's advance to TDC
and OR temperatures are too hight (which further increases pressures )
All this has been common knownedge since probably 1900 AD
In our model airplane ignition systems we are working with fairly low compressions and a fairly low energy ignitions that are pretty similar . The curves available are sometimes grossly over advertised in theitr value
I guess (don't know ) that the one in question, stays at low advancetill abour 3500rpm? the nshifts on up to max
ZDZ made some that had variable curve in order to minimize the typical out of balance shudder at 2500-3000 on singles
I used to measure the ignitions on a big ol SUN machine - then just stuck with cut n try to balance setups for thengine - w/wo tuned pipes etc..