I believe this is an important topic as a trainer to reach the participants from different cultural backgrounds.
Some nod to show affirmation, some sway their heads left to right and right to left to show affirmation, some cultures show affirmation by raising their eyebrows and some have some specific hand motions which show affirmation and again some only focus on words instead of non- verbal signals. Determining what target person/group/culture is represented will help us give the right affirmative signals and words towards the participants to keep the group motivated.