In the foregoing discussion the primary switch was opened thereby interrupting the current flowing through the transformer primary. The resulting collapse in the magnetic field will induce a voltage reversal in the transformer windings. The more rapid the field collapse is, the higher the induced voltage. The transformer will try to dissipate the energy stored in its collapsing magnetic field.
If the transformer was under load, the induced voltage would cause current to flow into the load. In the no-load case of this example, the transformer does not have any readily available place to dissipate the energy. The transformer will generate the voltage necessary to dissipate the stored energy, hence a high voltage kickback (or flyback or backswing) voltage will occur in the windings. In a real circuit the transformer will induce eddy currents in its core thereby dissipating the energy as core loss. In a real circuit the high voltages can damage the switching elements (transistors, F.E.T.s, S.C.R.s, etc.). Many designs include protective circuitry across the primary winding.