I have Tesla's books and in my opiniom main aspect of his transformers was over-unity power. Tesla design is flat spiral coil as secondary that allow create the asymmetry for magnetic fluxes from primary coil to secondary and from secondary coil to primary. The condition for over-unity is created here.
Free power is possible here.
Also Tesla did not used sine current from generator. Also power in load is taking from extra-coil that is placed in open wire but not in closed circuit. It is extra-coil in wire of antenn, as I think. Term free vibration explane the nature of free energy in such sort systems.