The cosworth boost control is quite simple,

ok, in laymans terms going right back to basics,
the turbocharger produces boost pressure, produced by the exhaust gasses driving the turbine wheel which in turn drives the compressor wheel,

now to slow boost down we need to prevent so much exhaust gas passing over the turbine, so we open the wastegate which bypasses the turbine, so it slows down, thus producing less boost pressure.

boost control simply has to control the operation of the wastegate itself, which will control turbo speed or boost pressure...

to do this we use a wastegate actuator which is pressure operated, ie at 1 bar it will open the gate and slow the turbo,

so if we want to run 2 bar of boost, we need to stop 1 bar of the boost pressure reaching the actuator, otherwise it will open at 1 bar! so we bleed some off through the amal valve. this valve is ecu controlled and can be opened and closed at different points in the map,

theres much more to itbut thats about as simple as i can make it this late lol

edit, i see i was beaten to it lol