The phrase "letted flow" refers to allowing something to move freely or to express oneself openly. There are several synonyms that can be used to replace this phrase, depending on the context. For example, the term "let loose" can be used to describe the release of something or someone from restraint. Another synonym is "unleash," which means to release or set free. "Pour forth" is another suitable synonym, which describes the act of expressing oneself or a feeling in a passionate or unrestrained manner. "Spill over" is also a good replacement for "letted flow," as it depicts something that is overflowing and unable to be contained.