Self-presentation
In fact, it is very difficult for students or teachers to change to Chinese to explain something that they get used to speak in English, like some technical terms. For example, Tutorial class, Lecture, etc. They are not showing off themselves. It is just a easy and natural way to express themselves. If their group have the same kind of education level or workikng status, people may get invloved into the conversation easily.
Face & Politeness Strategy
Some people think that using code-switching is a kind of face threatening to someone's positive face. For example, when you use some English code-switching when talking to a Chinese who doesn't know a bit English. What will your partner think ? He (or she) may feel very embrassed that he doesn't know what you mean. As a result, your action is threatening your partner's positive face.
Contextualization Cues & Frames
Some may think that the code-switching implys the change of frames. In some cases in a family gathering, when a student talks to his parents, sometimes includes some code-switching, his parnts may not understand what his children means with the code-switching. Then parents may change the topics or children are always lazy to explain. As a result, topic will be changed mostly after code-swiching.
Positioning & Cultural Storylines
People may think that when one uses code-switching, he or she may have a high social status with high level of education in the society. However, the fact is that if one is not really good at both languages, he or she cannot express himself (or herself) in exactly one language. He (or she) even needs another language helping him to express his words. As a result, the positioning of that person is just depending on how you think that person, not only depends on his varieties us of language.
Conversational Style
Some may think that code switching is just their own conversational style. It is not natural to force them to choose either one language to speak. They have their own choices to switch the language in a conversation. It is not for people showing off, but it is also their habit. Some people are born or studied in western countries, their style will be like "half Chinese, half Foreign". They speak naturally in merging two languages together. If it is a natural act, it is possible and acceptable.