An example would be this line from Wilfred Owen’s ‘Anthem for Doomed Youth’:
“Not in the hands of boys, but in their eyes.”
Now, when read aloud, it would sound something like this:
NOT in / the HANDS / of BOYS / but in / their EYES
The keen-eyed will notice that the first foot is a trochaic substitution, meaning that the ‘de dum’ rhythm has been replaced by a ‘dum de’, to emphasise the ‘NOT’. But the pyrrhic substitute can be found in the fourth foot.
Now, these syllables both need to be unstressed, because ‘but IN their EYES’ would not sound quite right and, more importantly, the emphasis of the word is completely unnecessary.
Similarly, if Owen had plumped for a trochee and had “BOYS BUT in their…” the result is actually quite ugly. So, because it is neither desirable nor necessary to have these syllables stressed, they are the unstressed beats of the pyrrhic meter.