BODY LANGUAGE IS OUR FRIEND

Human beings often physically telegraph what they are about to do, and over time one can get a feel for an actor’s movements and predict what they may do next, and where.
This works well with sudden movements. I’ve become very good at being able to tell how far an actor will jump, for example, by having an innate sense of how far they can move in one step for someone their height. If an actor is moving suddenly in one direction or another it can be easier to move the camera to where I feel their end point will be rather than try to follow them. For someone like myself, whose reflexes are okay but not great, I can make up for my lack of reflex speed with my excellent sense of timing. Instead of following fast action I often focus on properly timing the start of my move and then moving the camera a set distance at a set speed, based on rehearsals or gut feeling. This works very well, although it takes a certain amount of faith at times.
Most operators will tell you that the hardest shot to execute is someone standing up from a sitting position. This is absolutely true. Practice, practice, practice.