Intake pressure provided by the turbo or supercharger - basically exhaust gasses spin one side of the turbine that causes the other side to pump air in to the engine at higher-than-atmospheric pressure.
This allows for more fuel to be added for more bang per engine stroke!
It's not that. You only see the "BOOST" indicator when coming out of curves; if this were a turbocharger you'd see it all along the straightaways.
Someone else mentioned KERS (kinetic energy recovery system); that seems a lot more likely.