A picture sequence removes the two things that usually stop pupils speaking: not knowing what to say, and being afraid of being wrong. The pictures supply the content, so all the pupil has to supply is the language, and because there is no single correct wording, weaker pupils can contribute a sentence while stronger ones build a paragraph from the same image.
The six sequences are drawn from ordinary life rather than from a foreign setting, so pupils are not decoding an unfamiliar world at the same time as decoding English. Each one is deliberately open at the end, which is where the most interesting speaking happens.
The same sequence is used across four stages, from naming what is in each frame, to retelling in pairs, to a group version with connectives, to individual writing. Teachers with a single forty-minute period can stop after stage two and still have a complete lesson.