Understanding the slayer exciter circuit base voltage
Figure 1.


The main trick to understanding this is to think of capacitors as voltage maintainers - a bit like a short-term battery.
The parasitic capacitance of the coil to ground at (1) tends to hold (1) at a constant voltage.
When voltage is induced in the secondary the voltage across the coil rises.
If the top of the coil - the positive end - is held at constant voltage then the voltage at the bottom of the coil will be driven in a negative direction. This will continue until (2) reaches about -0.7 V at which point D turns on.
D has a much lower impedance than C does so now (2) is held at -0.7 V.
The still-increasing coil voltage therefore drives (1) more positive.




Figure 2. Slayer Exciter simulation done by LTSPICE. Source: ElectroBoom which explains the operation in further detail. (Click image for full resolution view.)