OK, this is how I illustrated the effect of negative offset to a friend I was attempting to explain it to in my shop one day.
Take a bucket and turn it upside down on the floor.
Take a broomstick handle and place it center of the bucket.
Apply about ten pounds of effort, pushing straight down on the top of the stick and attempt to rotate the bucket on its side.
Obviously, nothing is going to happen.
Now, tilt the top of the broomstick about 5 degrees, and apply the same force...boom, the bucket flops right over.
Now, the further you tilt that broomstick, the less effort it takes to knock over the bucket.
This is exactly what is happening inside the engine in relation to piston pin offset.
The factory offset keeps the rod completely vertical for a couple of crankshaft degrees beyond TDC.
Reversing the offset has the rod leaning into the direction of crankshaft rotation at TDC, and so the initial pressure pulse is applied to rotation that much sooner.
With an engine like the slant, having such tiny workable area at the base of the combustion chamber, this effect becomes even more critical than it would be in an engine with a more square bore/stroke relationship. literally every degree counts...there just simply is not enough volume available in the chamber to effectually work the piston for the entire length of the power stroke.
Make sense?