gurning

a consequence of the radiotherapy treatment is that it can have a detrimental effect on the health of your teeth and jaw.

To try and counteract the damage to the jaw which can lead to problems opening your mouth to it's full extent post treatment, you are encouraged to perform some mouth exercises. Basically this means opening and closing and twisting your mouth in a yawning action about 10 times, 3 or 4 times a day.

To summarise, it's gurning.