The posterior bellies of digastric muscle are especially active during
Digastric

Origin – The posterior bellay arises from the mastoid, or digastric, notch immediately behind the mastoid process of the temporal bone.
Insertion – The posterior belly passes downwards and forwards towards the hyoid bone where it becomes the intermediate digastric tendon and joints with the anterior belly.
Nerve supply – Derived from the digastric branch of the facial nerve.
Vasculature – Arterial blood supply from the posterior auricular and occipital arteries.
Action – The muscle depresses the mandible and can elevate the hyoid bone. The posterior bellies act in unison and are particularly active during swallowing and chewing.
Swallowing and speech
Swallowing and smiling
Swallowing and chewing
Swallowing and frowning