Precise focusing can be big challenge in street photography and in most of the sports and adventure activities. subjects generally move so fast that autofocusing systems fail to keep a track and lock on the fast moving subjects so many times. One way out is to set the focus before the subject even enters in the frame or reaches the point when the shutter would actually be clicked. In other words, you may release the shutter just before the peak moment so as to actually record the peak moment. This is known as Predictive focusing or pre focusing.
In many sporting activities, every participant follows the same track around a course. When you prefocus, rather than wait for an athlete to reach a particular spot, you focus on that spot in advance. You can then adjust the lens to precisely the right position—either by using manual focus control, or using autofocus and then locking the focus at that point. You may also autofocus at that point and then simply switch the camera to manual focus. However, remember to switch it back to autofocus once you are done with the situation. When the subject reaches exactly at that particular position in the frame, you release the shutter—and a sharp picture is instantly captured on the sensor.
New digital SLR cameras have a focus mode that tracks fast- moving subjects. “Predictive autofocus” adjusts the lens
even after the shutter has been pressed to predict where the subject has moved during the short delay before the exposure actually starts. Depending upon the Camera model and the lens being used this would be precise and accurate. With less advanced equipment, even if present, may not work very well. In this case the priority would be given to focusing compared to shutter release and this setting can be changed as well. When changed, the priority would be to release the shutter even if the correct focus has not been achieved.
This is called Servo / AF servo mode on SLR cameras.
While panning it is a good idea to pre focus at a point or rather the distance along which the subject intends to travel /move.
Pick up the right spot at the prefocused distance, where you feel the composition would work the best, when the subject is added to it.