In general, I think participation ratio would pretty much always be high with Turn and Talk, especially if it's used frequently throughout a lesson to break up instruction and brighten lines. Think ratio, on the other hand, is a little unpredictable with Turn and Talk. In my experience, short discussions between small groups of students do not always produce thoughtful analysis, and since there is only one teacher, monitoring students during these discussions can be difficult. To amplify success, a teacher might choose pairings ahead of time to avoid unproductive groups, and she also might move from Turn and Talk into an activity that keeps groups accountable for their discussion time: Groups might be cold called to share their responses, and those responses could be subject to whole group analysis and revision.
In Turn and Talk, I would want student pairs to practice good Habits of Discussion that build cohesiveness of ideas from one to the next and that prompt students to give evidence for reasoning. To prompt these behaviors, I might say something like "Remember to listen thoughtfully to your partner and build on his ideas. Do you agree? Disagree? Why? Make sure you each give specific reasons for your answers." I would probably use a simple in-cue, like "Okay, now turn and talk. Go!" A countdown would work well for the out-cue to allow students to wrap up their thoughts: "Turn back to me in three, two, one." Before talking, I might ask students to spend a minute or two summarizing the fruits of their discussion in just one sentence, like Art of the Sentence.
Honestly, I struggle with participation ratio in my lessons. I have so much to share with students and usually just one class period to do it! So while think ratio is very important, initially I would seek to boost participation ratio with Turn and Talk. I might ask students to discuss attributes of effective search terms during Turn and Talk. I could boost thinking afterward by sharing students' revised search terms, asking students to analyze and revise together before revising their own search terms.