You bend over backwards when you put yourself in an uncomfortable position in order to help another person. Example: "We bent over backwards to help him, and he never even thanked us!"

It is difficult to bend over backwards, and it feels awkward, but you are willing to do it to help certain people. Example: "My dad knows that I would bend over backwards to help him if he needed it."

At other times you are only willing to help when it is convenient for you, and you would not bend over backwards. Example: "I will help you out if you need it, but I am not bending over backwards for you."