If you've ever had to take a phone call in a loud place, a restaurant for example, and you find yourself leaning over, head down, plugging the other ear, closing your eyes or even walking into a dark corner, you are trying to encourage sensory compensation. By 'blocking out' visual, physical and other irrelevant input, you are trying to decrease the resources your brain uses to process those inputs,  and increase the attention/resources to the ear on the phone- in the hopes that you'll hear better.

 

Attention is not an infinite resources. It runs out. The strategy above mimics what the brain already does naturally.