"Hand a pen and a piece of paper to a member of the audience. Ask them to write a word, number or name on the piece of paper. As they are writing, you instantly know what they have written. Your back is turned. You never see the paper. There are no peeks, no gimmicks, no sleights. You do this purely with your own skill.
How is it possible?"
If I remember correctly there was a slate sold in the 1930s that was treated so it would make more noise when someone printed something in chalk. It was to help with sound reading.
I imagine this would take a lot of practice and would rarely be completely reliable. So many other ways that are more consistent in their results like the little device Dunninger used to hide while smoking a cigarette. That would always work.