The mind attends to that which makes a powerful appeal to the senses. For example, A sudden pause, with lifted hand, will stop confusion and cause students to listen and give attention. A picture or other illustrative material will arrest students' attention. A sudden raising or lowering of the voice arouses fresh attention. A pause in talking gets attention.
Silence can shock students into attentiveness.
A sudden noise can awaken sleeping minds. These are just devices to use when necessary to get attention. A teacher's effort at all times is to make his presentation so interesting that the students will listen and follow it. Teach students to concentrate. Move them from compelled to attracted attention.