Implicit feedback in human–human conversaton conversation is when small facial or bodily movements, or things we say offer the initial speaker feedback as the whether they were understood. Explicit confirmation, such as "yes"" or "you said you want a ticket", can be cumbersome. Instead a response such as "which concert would you like a ticket for", provides Implicit feedback that the the second person heard a request for a ticket, but also carries the conversation forward.
Used in Chap. 19: page 307