Why was Shakib Al Hasan biting a string while batting against India?

Bangladesh all-rounder Shakib Al Hasan was biting a string while batting in the first innings in the Chennai Test against India. Cricketer-turned-commentator Dinesh Karthik explained that the string acts as a self-check mechanism. It helps Shakib keep his head straight and does not allow it to drop towards the leg side, Karthik stated. Shakib earlier struggled with vision problems.