An important part of this is the response that draws your attention to unexpected or unusual stimuli. These might, for example, indicate that a predator is about to strike. In a natural environment, this drawing of attention is very important for survival. However in a modern sporting environment these are distractions that break flow. Loud noises can come from cheering crowds.
Flashes of light can come from flash photography.
Movement can come from performers in unrelated events, etc. Part of learning flow is learning to isolate the important stimuli for the sport from the irrelevant ones that cause distraction. This will involve learning to selectively override your brains natural reaction to stimuli.