At the simplest level, teacher talk time (TTT) refers to how much the teacher talks during a lesson. However, this will vary according to the stage of the lesson. For example, the teacher needs to speak more when providing explanations of and examples for the target language early in the lesson. Elsewhere he may speak less as students need ample opportunity to practice the new material. Overall, however, the teacher should roughly limit his speaking to 20% to 30% of the class time, with the remainder devoted to speaking/use of the language by the students.
On the other hand, Student Talk Time (STT) should be around 80% during the course of the lesson. Their use of the language should further promote qualitative thought. For example, this means that choral drills, substitution drills, and other exercises remain important because students need these activities to become familiar with and absorb the target language. However, too many drills or other, similar activities result in students who switch off their brains. The fail to critically observe, analyze, and practice with the new language.
Talk time by the teacher and students accomplishes the following:
1: It allows the teacher to restrict his speaking to vital areas of the lesson. When he then speaks, students know the information is important. They listen more attentively and work harder to successfully process the information.
2: Students get to speak more. When students speak more, they have increased opportunities to become familiar with the new material.
3: Students have more chances to experiment with and personalize the language. They can mix previous vocabulary and grammar structures with the target language of the lesson, as well as steer conversations towards their individual interests.
4: As students speak more, they must also rely on their skills. For example, if two students fail to understand one another, they must work together to repair the miscomprehension. This better prepares the class for the real world, where they can't rely on the teacher for help.
5: As the teacher speaks less, students have added opportunity for interest and challenge. For example, think back to your life as a student. Which classes did you enjoy the most, ones with a long lecture or ones that allowed active engagement?
From the above five points, it should be clear that the class greatly benefits from limited talking by the teacher. What's more, these are but a few of the positives available with low TTT.