The phrase 'tenses up' implies a situation where one becomes stiff, rigid or uptight. The antonyms for the phrase would be words or phrases that describe someone being relaxed, calm, cool, and collected. Synonymous terms that could use as antonyms would be - loosen, unwind, slacken, calm down, lighten up, let go, take it easy, de-stress, chill out, and so on. These phrases reflect motion or emotionally relaxed behavior instead of stiffness or anxiety. It is essential to recognize when we are becoming tense and utilize the appropriate phrase to return to a relaxed and composed state.