How does this current limiting circuit work?
There is a missing bias resistor from T1s collector to base:

The full circuit should look like this:







simulate this circuit &ndash; Schematic created using CircuitLab

[Update] Added calculation for bias resistor:

The bias resistor maximum value is given by \$R_{Bias} = \frac {V_{in} - V_Z} {I_Z + I_{B(max)}}\$ where \$I_Z\$ is the mimimum current to maintain the zener voltage and \$I_{B(max)}\$ is the maximum base current required for the pass transistor at maximum load.

In response to a comment: \$V_{out}\$ is given by \$V_{Z} - V_{beQ1}\$ provided the circuit is not in current limit.