I'll take a shot at this...
When the exhaust gases go through the turbine of the turbo, they can go one of two ways. They can either go through the turbine itself, or they can go around the turbine and through the wastegate hole. Any exhaust gas that bypasses the turbine and goes through the wastegate hole is "wasted" (i.e. not used to drive the turbine), thus the name "wastegate".
When boost is low, the wastegate puck covers the wastegate hole, forcing all of the exhaust gas through the turbine. The puck is spring-loaded by the wastegate actuator to stay closed. As boost increases, two things happen:
1. The pressure of the exhaust gases acting on the puck increases (i.e. the exhaust gas is trying harder to push the puck open).
2. The canister on the wastegate actuator is filled with pressurized boost air from the compressor, under the control of the wastegate solenoid on the valve cover.
Both of the above try to overcome the spring in the actuator and force the puck open. So, as your turbo is building more and more boost when you floor it, the pressure of the exhaust gases and the boost in the actuator force the puck open. This allows some of the exhaust gas to bypass the turbine and go through the wastegate. This prevents the boost from building beyond a certain point.
So, to raise boost, you either stiffen the actuator spring or decrease the amount of boost pressure going to the actuator via the wastegate solenoid. To lower boost, you either make the actuator spring less stiff or increase the amount boost pressure going to the actuator via the wastegate solenoid.
Clear as mud?
