It's turbo spool. Stomp on it when exhaust gas is plentiful, say above 3500 rpm, and you get power FAST. But stomp on it when you're down below 2000, say, and you have to wait, wait, wait for the turbo to spin up. If you roll on the throttle, the turbo can spool as you sqeeze on the gas, and it feels like there's less waiting.
Really, if you haven't driven other turbo cars, the Legacy has GREAT turbo response. Even a stock WRX has MUCH more turbo lag than this car.