You don't mind talking while they talk. You are willing to speak louder so that they can finish their conversation even after you have have started the lesson. They get the idea that you accept their inattention and that it is permissible to talk while you are presenting a lesson. The focusing technique means that you will demand their attention before you begin. That you will wait and not start until everyone has settled down.
Experienced teachers know that silence on their part is very effective.
They will punctuate their waiting by extending it 5 to 10 seconds after the classroom is completely quiet. Then they begin their lesson using a quieter voice than normal. A soft spoken teacher often has a calmer, quieter classroom than one with a stronger voice. Her students sit still in order to hear what she says. Uncertainty increases the level of excitement in the classroom.