the 2 stage boost is very simple - it's just a switch to the power of the boost control valve (common misnamed the amal valve as it is made by a company called SCM in
Åmål in Sweden and people confuse this with the british company of Amal that used to produce carburettors and such in the olden days).

the boost control valve works by allowing boost from the compressor to reach the actuator or not (via a bleed off orifice). by altering the power to it, the ecu can allow the boost to be reduced if there is a problem with a sensor for example. the 2 stage boost control switch just replicates that function manually.