Boost controllers control boost level by affecting the air pressure delivered to the wastegate. This can be done by manual, electronic or full out engine management systems. In this section you'll find the electronic and manual type.
Without a boost controller you'll be limited to how much boost your turbo is capable of by the spring that is set at a predetermined safe boost level. The boost controller alters the incoming charged air to the wastegate to allow it to open earlier or later, thereby changing the amount of boost your turbo provides to the engine.