Final answer:
The subject of this question is Physics, specifically the principle of Pascal's principle in fluid mechanics, which explains the operation of hydraulic brakes.
Step-by-step explanation:
In the context of the question, the subject being discussed is the principle behind the operation of the brakes. This falls under the subject of Physics. Specifically, it relates to the principle of Pascal's principle in fluid mechanics.
When the driver lets the air out of the brakes using the parking brake control, the air pressure in the brake system is reduced. This affects the operation of the hydraulic brakes, which rely on Pascal's principle to function. According to Pascal's principle, when pressure is applied to a fluid in a closed container, the pressure is transmitted equally in all directions. In the context of hydraulic brakes, this means that when the air pressure is reduced, it allows the springs to apply the brakes.