Left of thumb usually (but not always) puts the MB very close to the track, if not in the initial track. The MB will have its own migration, just like the pin does, so just as putting the pin in the track will essentially kill any flare potential and a significant part of the ball's reaction, so will putting the MB there. Basically you're taking the effect of the MB on ball reaction out of the ball's, umm, reaction.
It's more pronounced in the newer, strongly asymmetrics like Storm's RAD balls, Track's newer Robo Core balls (Machine family), Morich's Vanguard balls, and The One series from Ebonite. Putting MB so far away from the PAP and close to the track can also reverse the flare, making the ball track over the finger or thumb holes. Older, weaker asyms that don't have a 0.02 intermediate diff (pre-Vanguard Morich, most Morpheus-cored Track stuff, a lot of Ebo's older stuff) don't suffer that problem to the same extent. Those have an intermediate diff in the 0.01-0.015" range and higher spin times (7s or more).
If you want to take the effect of the MB out of the ball's reaction, get a ball that doesn't have a real MB. Buying an asymmetric ball (and especially paying the price for that technology) and removing the effect of the asymmetry seems silly to me.
SH
