It's not the breaks. lol.
He asked you to keep foot on throttle and then cleaned the inside of Throttle body, which has a sensor fed flap to regular air and is quite powerful, when the self is on ON position, power is fed to Throttle body and it can snap the fingers if not careful.
Most of the vitz rough idling comes from poor maintenance of TB. Because IKR-FE is weak engine with only 3 cylinders, the performance of the car really comes from the CVT which also needs to be maintained and CVT oil changed every 35-40K kilometers.
It's a common sign that you need to clean the TB if car is rough idling too much or it's louder than usual. CAT Converter, TB, Air Filter, Intake Manifold, Spark Plug coil connectors, Injector connectors, all need regular cleaning to maintain a smooth running Vitz. Once a month, dedicate your 4-5 hours for this.