[![enter image description here](https://i.stack.imgur.com/tErBz.png)](https://i.stack.imgur.com/tErBz.png)

In "switching power supply handbook" by Keith Billings P2 is supposed to represent leakage inductance and D3 is a catch diode, but i don't get quite right.
What is that diode for? how is it even connected to the leakage winding while this winding physically doesn't exist?
He also said that this diode connects when the voltage at the collector reaches 2VDC but I don't see how would that happen.
What am I missing here?