Help me to understand this schematic
With no load resistance, no current flows out of the 7812. No current flows in, either, therefore the voltage drop across R7 is almost zero. The bases of all the transistors are held at the same voltage as their collectors, so they do not conduct.

As current is drawn by the load, the current out of - and into - the 7812 increases. This causes a voltage to be dropped across R7. As the load current increases (to around 7mA in this case), the voltage across R7 reaches 0.7V, and so the transistors begin to turn on and conduct. Their collectors are always held at the output voltage of the regulator.

As the transistors turn on, they start to 'divert' the current drawn by the load away from the regulator. The 7812 does not have to be able to conduct much current at all, as the larger the drop across R7 the harder the transistors are turned on. Eventually an equilibrium is reached, usually with the 7812 only making up around 5% of the current supply. However, this figure depends on values of R1-6 and their ratio to the value of R7, plus the characteristics of the particular transistors.