(Source: Zahr al-Adab wa Tsamar al-Albab, Vol. 1, p. 148)
Hadaju bi-asma'ihim, literally "they aimed their hearing at you," lays one duty on the speaker: to read when the eyes before him begin to go out.
"A speaker's vigor is measured by the listener's understanding. 'Abdullah ibn Mas'ud (ra.) used to say: speak to people as long as they turn their hearing toward you and fix you with their eyes; when you see slackness in them, hold your tongue."