Tech Question....Adjusting Turbo Boost????

[GASWAGON]

O.k now i'm a Diesel mechanic by trade so i know exactly how simply any turbo works on a truck engine.

But, My question is about the Good old Barra I6T engine.

How exactly is the boost 'adjusted' on say a stock engine from say 2-3 psi up to 10, 15, 20psi and so on to pump out all the power you guys have been getting out of them?

Is it just more fuel which equals bigger explosion/burn???

I also realise bigger and different designed turbo's also help with the power gains.