She literally has tunnel-vison when it comes to her objective.

im guessing shes saying how the brain fills in the details for stuff you see in your periphery, and she's using it to see through her shirt

I think she's saying she can see Ai-chan's face AND chest simultaneously, given the way she expresses her eye's "up and down" periphery at the end. Normally when you're making eye contact with a woman in conversational distance, she's able to tell if your eyes are glancing at her chest, because of the traditional 30 to 60 degree limitation. Volley-bu-chan got around this by training her eyes to see beyond that. Very helpful for playing sports of course... but more so to ogle giant marshmallows.