Good question.
What it's all about is the fact that when a bullet begins to move into the throat and into the barrel, a plug of the propellant follows it and effectively acts adds to the mass of the projectile itself.
It seems the default value in QL is usually 0.5 (which I take to mean that 50% of the powder charge is effectively projectile mass for an instant), but I quit paying much attention to it. If you play with it you can see it does have real effects...but how to adjust it to some "more correct" or precise value is a trick I figure might break my neck. I don't mess with it. I've come to the conclusion that QL outputs tend to be relative anyway, and not that it's not useful---it's just not going to provide any reliable absolutes.
Thanks Riverrider, you use it the same way I do, that is “leave it along”. You did do an excellent job of explaining what it represents. I recall the manual said you could increase or decrease the value on certain bullets. My manual is for Version 3.0, some of those bullets are gone or out of vogue now.
I honestly don't recall what the manual in my version says...I think it's 3.2 but I am far from sure. Maybe I should have another look at it myself...I've probably forgotten everything I read!