Someones it’s the gas pedal that it turns out was only half-used before, sometimes in a fight the punches only become way stronger when the protagonist is already tired, beaten, and about to collapse, even bullets sometimes start working on the big baddies from point-blank range only when the protagonist is about to die but then drops a one-liner.

I can maybe believe the fist fight one. Adrenaline kicking in even more or whatever. But I do think it suggests writers haven't been in many fights themselves and are just armchair sciencing how fighting works.

And the bullet one and gas pedal one is indefensible