transistors - Understanding the slayer exciter circuit base voltage

I am quite new to electronics and I have a bit of trouble understanding part of the slayer exciter circuit.

If we take this schematic from electroboom here:

I don't understand how the secondary would send a negative voltage to Q1. From the flow of current through the primary, it would seem to me like it would actually send positive current; I don't see at what point Q1 is interrupted.

How does the secondary create a negative voltage?