This is a brake disc spin test. Getting to 1,000 mph is relatively easy, slowing down safely before you run out of desert is much more of a challenge. At 1,000 mph there will be 15 tonnes of drag pushing against the car. When the jet and rocket engines are switched off BLOODHOUND will rapidly decelerate to 800 mph at which point the air brakes will start to be deployed.
They will open out into the airflow and maintain 3G deceleration (the equivalent of you going from 60 mph - 0 in one second - for 20 seconds). Once the car slows to below 200 mph, driver, Andy Green will use the more conventional wheel brakes to bring the car to a halt.
This video shows the brake disc testing at AP Racing: