Does "squinting" make you concentrate better on a logic test?
The phenomenon in question is probably related to people with uncorrected/undercorrected myopia.

This "ancient" phenomenon is called stenopeic slit effect (a case of pinhole effect): when the person squints the visual acuity becomes better because of smaller blur circles: the "slit" between the eyelids is the key point improving blurring. 

One can check if it works by creating slit in paper or plastic:



Some "charlatans/witch-doctors/quacksalver/etc" can "treat" refractive error with pinhole/stenopeic glasses:



It is known that children with refractive errors have attention issues because they cannot see the objects sharply (this is why they can be mistakenly diagnosed with ADHD), thus your concentration improvement related mostly to your vision improve and not a better "understanding".

If you mean by squint using mimic muscles and corrugator muscle the answer should point out that not a "squint" improves your concentration, but your attempt to improve your concentration causes this type of "squint".