You electrically stimulate around the spot with a probe where you’re planning to resect. If the patient can’t perform whatever task you’re evaluating then you know you can’t resect in that area. Generally looking for a 1cm margin between tissue you’re going to resect and a positive stim site.

Can’t undo brain cuts. CNS neurons don’t repair themselves like peripheral neurons or your skin. Generally not cutting glioma brain tumors out per se more likely to use ultrasonic aspiration to suck the tumor out piecemeal. Depends on the tumor though.