In “Shutter Priority” mode, you manually set the camera’s shutter speed and the camera automatically picks the right aperture for you, based on the amount of light that passes through the lens. This mode is intended to be used when motion needs to be frozen or intentionally blurred. If there is too much light, the camera will increase the lens aperture to a higher number, which decreases the amount of light that passes through the lens. If there is not enough light, the camera will decrease the aperture to the lowest number, so that more light passes through the lens. So in Shutter Priority mode, the shutter speed stays the same (what you set it to), while aperture automatically increases and decreases, based on the amount of light. In addition, there is no control over subject isolation, because you are letting the camera control the depth of field.
I try not to use this mode either, because there is a risk of getting an overexposed or underexposed image. Why? Because if the amount of ambient light is not sufficient and I set the shutter speed to a really high number, my exposure will be limited to the aperture/speed of my lens. For example, if the maximum aperture of my lens is f/4.0, the camera will not be able to use a lower aperture than f/4.0 and will still shoot at the fast shutter speed that I manually set. The result will be an underexposed image. At the same time, if I use a very slow shutter speed when there is plenty of light, the image will be overexposed and blown out.
With this in mind I took advantage of this and so used the concept of movement with slower shutter speed to take this shot :).