By looking at the Current Wave Forms, one can see what's going on in the Core, Hysteresis, this is Electromagnetic Induction.
Below, in light Green, we see the one of the Output Current Wave Forms:
The wave itself looks like this:
You can see, the Current changes Polarity, Very Fast, almost at Mid Cycle: (Green Arrow)
A lot os questions can be asked, most all have very simple answers!
From the Peak of the Current Wave form, where it Very quickly changes polarity, to the actual change in polarity, is about 4 Micro Seconds. Very fast!
Because we know that Graham Gunderson uses Partnered Output Coils, and we know that the Coil being measured, Flips polarity, very quickly close to the Peak Current, we know at this Point that the Magnetic Fields Oppose, as the Currents must also Oppose at this point, between the two Output Coils.
Graham is holding the Coil Polarity in one Direction, until close to Top Dead Centre of the Current Wave Form, then Dropping the Polarity with a Switch, a Mosfet. This circuit we know already:
It turns out, this is a similar method of Operation as Bradleys RT. This is a type of Resonance, where we are bringing about, by means of a Timed Response, Maximum Electromagnetic Induction for Minimum Input. The Input is doing work on the Core, and getting Electrons Flowing in the Coils, but then the Polarity Flips and this is when the Magic Happens.
Note, this is a 1/4 Wavelength, you may see some Correlation to this Image now:
You can read more here: Electromagnetic Waves if you want more information.