We present "doing language testing" sequences; L2 learners decide for themselves to test themselves or each other explicitly on new linguistic items, outside the official task cycle with no professional present, investing extra time and energy. We examine how and why pairs of learners do this, and its impact on their learning. They use an App to learn Chinese language and culture whilst cooking in their university dormitory kitchens, receiving multimedia instructions and help from a tablet. Using a mixed-methods research design, we asked: How is the practice of "doing language testing" organised in interactional terms? Using multimodal CA we found: learners organised the interaction themselves and introduced their own learning interests. Some self-tested and some peer-tested; some consulted the system and some did not. To ascertain the reasons why the learners decided to "do language testing," we used post-hoc interviews and found their major motivation was to have an improved learning experience. We then compared their vocabulary post-test score gains with those who did not "do language testing" and found they made significantly higher gains with a learning advantage. The study shows that some students are motivated to "do language testing" in order to enhance their task performance and learning experience, in which they succeed.