Slayer exciter circuit - struggling with how it works

[Tatranka]

I'm trying to understand, How slayer exciter works, but there's some stuff I can't understand.


First, I don't understand, how induced current in the secondary coil and the capacitor can switch off the transistor. I'm trying to find out, what exactly happens inside, but most explanations are oversimplified. I've seen warnings some of them are even wrong.

I found one explanation by electro boom here https://www.electroboom.com/?p=521 where I struggle with points 4 and 5

Then, some of the circuits use a diode, but I found many schemes which don't use a diode at all.

Also, slayer exciter is famous for how it can autotune itself, but changing turns of the primary coil or secondary has effects on performance, so what happens inside?

And last, Tesla coil is a resonant transformer, bud I couldn't find any other explanation, of how it rises output voltage, then by the turns ratio of coils like a classic transformer. I know it can't be so simple and it has to do something with the resonance between coils.

I know my questions are quite chaotic, but I'm fascinated with Tesla coils, but I've tried my best a haven't found the answers I want. I would be grateful if you could help me.

[Twospoons]

I agree electroboom's explanation is a bit odd at points 4 and 5.  Its much better to think in terms of current, since a BJT is controlled by its base current, not its base voltage.
So at the BJT base we have a DC bias current from the resistor + the AC current flowing in the LC tank formed by the secondary winding and its stray C.
That AC current gets amplified by the BJT, and drives the primary coil, driving more energy into the secondary resonant tank. On each cycle more energy is added to the tank, making the AC current bigger, and the AC voltage at the output terminal bigger too.  Eventually the AC current is large enough that the BJT turns off completely for half of each cycle - so its running in Class C.  You can see why the phasing is critical - get it backwards and the amplified signal will oppose the oscillation, instead of enhancing it.
 The reverse diode provides a path for the AC half cycle that would otherwise be blocked by the base-emitter junction being reverse biased. Without the diode the current has to find another path, and might blow the BJT in the process.