I have not heard the "feathered in" part before.
Here is what I have written on the A9* use of Injector Breakpoint.
This is from stangnet as well.
http://forums.stangnet.com/showthread.php?t=640613
4. The A9L strtategy uses the Function - Injector BreakPoint a little differently than other strategies. For most strategies the Inj PB is the point at which the computer switches between the Injector High Slope and Injector Low Slope, which makes sense.
The A9L however is slightly different in that the Injector BP is actually used as an Injector PulseWidth modifier, and the Injector BP is hard wired into the EEC as 2ms. When you make a change to the Function Inj BP you are actually changing how much fuel is cut from the Inj PW below 1,500rpm. The use of a larger number in the Inj PB Function will pull more fuel from the already calculated PulseWidth, until 1,500rpm.
To the best of my knowledge this is correct.
Let me know if clarification is needed.
jason