Yeah, you use the switch to break the power to the amal valve. This fools the amal valve into thinking that the ecu has found a problem such as too higher boost temp and switches the amal valve to not bleed any air off (as it does in normal conditions all the time anyway). On standard boost setting of 7.5psi i think low boost is about 2psi as this is what was considered as a safe 'limp home' level boost. As you wind the actuator rod, this will obviously increase along with your high boost setting.

As mentioned before the system is a failsafe so by only putting a switch in you can only turn the boost downwards, and it won't effect the ecu's safety mechanism on high boost either (as the switch will not be seen by the system)