What is meant by the phrase "did not fill the mouth" in the context of the ruling on vomiting that does not break the fast according to the Hanafis?

Abu Yusuf holds that intentionally vomiting or swallowing vomit does not invalidate the fast if it is less than a mouth full. Muhammad ibn al-Hasan disagreed, and many scholars have preferred his view that the fast is invalidated by intentionally vomiting or swallowing vomit, regardless of the amount. Ibn Kamal al-Wazir considers Abu Yusuf's opinion weak due to its contradiction of the text, and "a mouth full" is defined as what actually fills the mouth and is considered to be like an external excretion.