One tries to find the discrepancies between the words being said and the emotions and feelings that are shown (eyes, facial expressions, body posture). Judges can only learn this through experience.
Being the judge, one needs to establish a common frame with the witness to be able to assess the truth. This common frame is often based on recognizing and accepting the witness emotions (anger, frustration, sadness, shame etc.) If these emotions are not accepted, the search for formal facts is hindered. The witness will not understand what the court needs to know, and the court therefore will not obtain the facts and understanding it needs.