As has been said previously in this thread, everyone has their own interpretation of the rule.
I sort of remember talking about this when I done my boat licence course, but that was 12 yrs ago.
My understanding is pretty much exactly what the signs say, NO wash, not minimal wash.
If your in a small tinnie doing 25 knots then you're not making much wash, but you are still making wash.
If you're just idling along with the motor in gear but no revs you're not going to create wash.
As the definition posted up said, it's about not causing a wash that makes the moored boats rock around and waves lapping on the shoreline.
With all that in mind I would suggest judging each case individually.
My 2 cents worth.