Though head-shaving is one of the compulsory rituals of both Hajj and Umrah, yet it is performed in a certain time. The pilgrims are not allowed to trim their hair until they reach a definite period. Trimming means shaving, scissoring, pulling or any other way to remove the hair. Since not a single hair is allowed to remove, so that's why combing is not permissible as it causes pulling the hair as well.