wow. this is going to be kinda hard to explain. a boost controller uses the boost pressure created by the turbo to fool the wastegate.

ill explain an external wastegate...
there is a vacuum line coming from a pressure source on the manifold that goes to the wastegate. this line feeds the pressure in the manifold/motor to the wastegate. when the pressure in the wastegate matches the pressure of the spring it forces the wastegate valve open a set amount determined by the spring.

a boost controller will force a set amount of pressure on top of the wastegate valve thru a fitting located on the top of the wastegate thus forcing it to stay closed until the pressure below the valve is greater than the pressure above the valve. when u turn the knob on a boost controller it just changes the amount of air being forced into the top of the wastegate.

kinda confusing, hope that helped a little