They can be compared to closed pairs, where all the learners work in pairs that do not monitor each other.
Example
The learners are working on developing telephone conversations using prompts. The teacher asks one pair to continue working while the rest of the group watches.
In the classroom
Open pair work can be a fast and effective way to highlight language learners might need for an activity, and clarify that people understand what to do. It needs to be managed sensitively, choosing confident learners to demonstrate, and dealing with errors through a "hotsheet"; a record of errors in performance, which can be discussed later.
See also:
https://www.teachingenglish.org.uk/article/closed-pairs
https://www.teachingenglish.org.uk/article/open-class
Further links:
https://www.teachingenglish.org.uk/article/working-pairs-groups
https://www.teachingenglish.org.uk/article/teacher-talking-time
https://www.teachingenglish.org.uk/article/my-english-telephone