Turbo lag is a defining characteristic of older forced-induction cars. These large turbochargers were used to feed high-performance engines and generate significant power. The problem would come when you called upon that power. Your right foot mashes the gas—then nothing. A shock of power would eventually make its way to the drive wheels, sometimes mid-corner. Turbo lag is notoriously finicky (and in the case of old race cars, a killer) and automakers have been working on ways to reduce or eliminate it. Volvo has its own solution and it involves a bit of engineering that we need explained.
The system is called Power Pulse, and it works with the help of an air compressor and an electric motor. The motor waits for the driver to ask for more power, and it keeps the air compressor tank ready to go with clean air pulled in from the intake. When the throttle is pressed, the air tank can then force compressed air into the exhaust manifold and send it along with the exhaust gases to spool up the turbo.
This is almost similar to what the Fast and Furious crew would be doing when they hit their "nitrous" button. They want to force more air into the engine, and here Volvo is forcing more air into the exhaust manifold to get the turbo up to speed more quickly. There's no need for bottles of NOS though, as the air compressor seems suited to handle the task.
Once the turbo has spooled up, the air compressor's job is done. Now the electric motor refills the tank so the Power Pulse system is ready to help out the next time it's needed.