This paper proposes a method to find clues to an understanding of C-language programming techniques from active diagrams which a teacher drew extemporaneously. An active diagram is a diagram which moves as time advances. A teacher often explains a programming technique with the movement of an active diagram. The method records active diagrams in a form that they can be reproduced afterward. The method collects also several kinds of data to grasp contents and effects of the active diagrams. The method picks out clues from the active diagrams from the data. Finally, the method provides the clues for all students eager to acquire programming skills. If they reproduce the active diagrams, they can learn effectively. We have developed data collection systems to realize the method. We plan to verify effectiveness of the method by application of the systems to an actual university course.