I’ve always had the headcanon that the Bloodstone Scepter gives the Dragonlord very limited reality-bending powers based on their force of will, which is why dragons are compelled to obey its wielder and why Torch was so massive.
By that logic, if Ember commands her partner to keep going long after their endurance should reasonably have run out, well… they might just get a second wind.
Gotta be a reason for all those dragon hybrids running around.