When you turn on a single-channel RC car, the wheels start spinning. They spin forward all the time, and when you press the button on the transmitter, they reverse and spin backward. So the car always moves forward, unless you press the button, when it moves backward and spins in a circle. They do this by changing the direction that current flows through the motor. The purpose of a diode is to allow electricity to flow in only one direction.
Electricity is blocked from flowing through a diode in the other direction.
Since a vibrator will run no matter what direction the electricity flows through it, the diodes are necessary to prevent the vibrator from running all the time. When you press the button, the flow of current reverses, the diode lets it through, and it runs. Turn on the power to the radio receiver from the car. The vibrator may start running immediately, even without pressing the transmitter button. If it does, flip the SPDT switch to its other position.