The phrase "bend back" is typically used when referring to the physical act of bending a part of the body backwards, such as bending back your arm or leg. However, there are several synonyms that can be used in place of "bend back" depending on the context. For example, you could say "flex back" or "arch back," both of which convey a similar bending motion. Another option might be "lean back," which implies a more relaxed and less forceful motion. Ultimately, the choice of synonym will depend on the specific situation and desired tone of the sentence.