Suppose I assert that no Scotsman puts sugar on his porridge. You counter this by pointing out that your friend Angus likes sugar with his porridge. I then say "Ah, yes, but no true Scotsman puts sugar on his porridge.
This is an example of an ad hoc change being used to shore up an assertion, combined with an attempt to shift the meaning of the words used original assertion; you might call it a combination of fallacies.
Sometimes the claim is legitimate, such as "No true Christian believes killing is moral" and therefore if someone believes killing is moral they are not a true Christian. Also, no true ALF member deliberately risks lives of other sentient beings.