if you dont have a BOV the turbo is pushing air into the motor at say 10PSI. When you take your foot off the accelerator the air has nowhere to go, so it tries to go back through the turbo hence the "HECTICT DOSES" of flutter.
It doesnt increase power, the air trying to go back past the turbo slows it down, therefore it could create more turbo lag because your trying to spin the turbo the opposite way.