A waste gate simply dumps exhaust gas pressure directly to the exhaust outlet or the atmosphere, rather than routing it through the turbine housing of the turbocharger to drive the turbine.
On modern engines, it is sometimes controlled by a solenoid, and is often operated by intake manifold pressure. It may be adjusted manually, or controlled by a computer.
On planes like the P-38, the P-47, the B-17, and the B-24, it was controlled by an oil pressure regulated system.
It was common to have problems with the P-38 and the P-47 especially at high altitudes, because pilots were bad about not controlling oil temperature. The oil would actaully congeal in the regulator, and the regulator would no longer function. the turbo would either build only about 5 psi with the wast gate stuck open, and provide no power, or it would build in excess of 80 psi, blowing the engine, the intercooler hoses, rupturing the intercoolers themselves, or overspeeding the turbo to destruction. The P-38 actually had an armor ring around the turbo to protect the pilot and plane from fragments.
All of those planes above shared the General Electric B-2, B-33, and B-34 turbochargers. The U.S. did not allow these systems to be installed on aircraft sold to toher countries.
