Final answer:
The motion technique being described is panning, where the camera is moved at the same speed as the subject, creating a sharp subject against a blurred background. This requires a particular balance of shutter speed and aperture, often achieved by using manual or shutter priority modes.
Step-by-step explanation:
The motion technique you're referring to is known as panning. Panning involves setting the camera with a specific shutter speed that is fast enough to freeze the subject while moving the camera at the same speed as the subject. Because the photographer moves the camera parallel to the moving subject, the subject appears sharp while the background becomes blurred, creating a sense of motion. Open up the aperture allows more light to enter, compensating for the fast shutter speed, which reduces the amount of light that reaches the sensor due to the shorter exposure time.
When shooting in manual mode, the photographer controls both shutter speed and aperture to achieve the desired balance of motion capture and depth of field. Shooting in shutter priority mode or using the sports scene mode might automatically adjust the aperture to work with the fast shutter speed required to freeze the subject. Using a larger aperture results in a narrow depth of field, which can enhance the panning effect by keeping only the subject in focus and the background more blurred.