Unaffected hemisphere gets a beneficial boost in brain BDNF and more.

From the article:

The uninjured side benefits as well with a boost of growth factors [like BDNF] and maybe even more blood vessels, possibly readying itself to take on more responsibility. “It’s still unclear what that hemisphere is really doing,” Fagan said. “Some people think it’s just suppressing abnormal neurons firing in the damaged area. We think it’s more active recovery.”