Nettle's poem puts us inside the mind of an animal, apparently a mother, at the moment a confrontation turns dangerous. An opponent rises up to its full height, and the speaker copies the move. Both are ready to fight, and the short, physical details (claws out, fur bristling, teeth bared) make the tension easy to feel.
Then comes the turn. The shouted "NO!" breaks into the poem as the speaker realises her cub is caught between the two of them. Fighting no longer matters; protecting the cub does. She lunges forward, grabs the cub in her mouth, and runs. The title line, "Fight, No, Flight," shows this change of mind, with one word sliding into another.
The mood starts tense and alert, then becomes fast and urgent. The short lines and lack of punctuation give a sense of speed, as if there's no time to think. What the speaker feels most is fierce, instinctive love, strong enough to outweigh pride or the urge to win.