This is a flawed concept which idealises the extent to which teachers can keep in touch and "control" what is happening when assistants and children work together in the classroom.
When teaching assistants are with one pupil, a group of pupils or, indeed, a whole class, spontaneous pedagogic relationships often develop.
This is because children tend to interact with any adult who works with or "supervises" them; and people become teaching assistants because they enjoy interacting with their pupils.
Faculty of education and language studies
The Open University