What causes the RPM's to drop when you apply brakes?

When you apply the brakes, the vehicle's speed decreases, which in turn reduces the engine load. As the engine encounters less resistance, its RPMs drop because the throttle is often reduced to maintain speed or decelerate. Additionally, the braking system creates drag, causing the engine to work less efficiently, leading to a further drop in RPMs. This is a normal response in vehicles, especially in automatic transmissions where the engine adjusts to changes in speed and load.