Here ya go, Dave.
This Airy Disc image was stolen from the web to illustrate how Al's Collimation Aid works. Unfortunately, the collimation is pretty good (perhaps a tiny bit of astigmatism
)... a poorly collimated Airy disc would probably illustrate it better.
Perhaps just to explain...
Lets say we aligned the inner circle with the inside of the Airy disc... if the scope is out of collimation the centre dot will be offset and not in the centre of the centre circle or cross hairs, and the outer edge of the Airy disc will be offset the other way relative to the outer circle when its adjusted to the right size. Clear as mud?
Al.