Matter. Mana. Energy.
These are three different things, for all that human intuition often gets them mixed up.
The body is made of Matter and Mana. It uses Energy. But before it can use that energy, it has to get it from somewhere.
As Hailey moves through the forms of her strikes, the force of her body is opposed by the force of the world. The shock of impacts, the recoil of her weapons, the heat generated by her muscles, the chemical reaction that comes with every breath. None of this goes to waste, as it would in a more entropic universe. The mana in Hailey's body absorbs this energy and moves into a state of excitation, storing that energy for later.
Imagine a flywheel, spinning faster and faster as one pours more and more kinetic energy into it. Imagine that flywheel drives a shaft on which there are seven gears. Four of those gears have teeth shaped to catch an aspect of Matter. Three more have teeth shaped to catch an aspect of Mana.
This is a poor analogy, prone to causing confusion in less-studious children, who often make the mistake of taking the analogy too literally and concluding that chi is the same thing as orbal energy, which actually involves spinning gears and the math of orbits. Chi very much isn't that. But, poor as it is, the analogy is compelling.
All that metaphysical momentum waiting to be used. Hailey throws out a wind blade, briefly engaging her metaphysical, metaphorical 'clutch', grinding the momentum of the mana in her body against the mostly-static elemental field constants, warping reality to make the Air sharp. She spends chi, giving up some of that stored momentum, to shove on the fabric of existence and shape it into something interesting, however briefly.
Hailey has a lot of practice at this, so she can shove her chi against reality quickly and precisely, draining less momentum from her body's mana than most girls her age would need, to achieve the same effect. She's far from perfectly efficient, though. Class Seven has only just started the lesson on doing the math, working out the actual energy necessary for a given effect and calculating the metaphysical 'mass' of their own mana and from that working out what percentage of their maximum 'momentum' they actually need.
None of them are above 20% efficiency, but 10% efficiency is impressive for a layman, so for teenagers they're all pretty far ahead of the curve already. The hard numbers mean little, Instructor Sara reminds them, though. Being efficient with the raw energy of chi is only one small part of the infinite possibilities in bending reality to one's will. Skill wins, every time.