It's a joke by River. "Turning off the brakes" could possibly mean switching the stealth landing gear on. Think slamming the handbrake on a car and that creaking noise it makes. That's the TARDIS materializing. River just put the handbrake up slowly.when the Doctor flys it he is pretending to like the noise to River Song it means nothingTheir TARDIS' do make the noise, so I guess all Time Lords leave the brakes on.
Even TARDIS' being used in an official capacity by non-renegade Time Lords make that noise.
Some fans have concluded that landing without the brakes is dangerous and difficult, a form of "hot-dogging" that whoever taught River came up with as a way of showing off.