The way the Snap-On timing work is that it has a variable timing capability.
Regular timing light simply shine when spark plug is ignited... stating the
timing at that point.
With Snap-On, you can manually set it to any degree you want regardless of
the engine's ignition. So, the light pulses is not in sync with the engine's
ignition... That way, you can check, if you set timing light to 10 degree
BTDC, the engine should also read the same... if you set it 180 degree, it
should read 180 degree.
You mentioned that the distributor position can be adjusted to chain wear...
Thank you for bringing it up... I am thinking... you are right when you set
it at idle or whichever MB specified...
However, with timing chain stretch, what you set at idle isn't the same at
180 degree, so that's where this adjustable timing light gun comes into
play... it can verify how far off you are.