Unfortunately i cant figure out how it works, could someone shed some light on the circuits working or some resources which would help understand its working. Thanks very much.
The 5.1V zeners has low impedance and fixes the Vb and thus Ve voltages with 0.65V drop for Vbe.
Thus the voltage drop on the emitter resistors determines current limit.
The 2k R's supply current and Vin removes current thus modulating the output to load current as a voltage controlled current source