To say that one is stuck "between a rock and a hard place" stresses that there are two forces which are restricting one's movement. Often it is the opposite direction of these forces that causes the sticking point. One is unable for some particular reason to do something that one would like to do, such as act or not act in a certain way. "I'd like to help you but I am stuck between a rock and a hard place." To have someone between a rock and a hard place is to have them in a position that causes them to make a choice that they don't want to make. "You've got me between a rock and a hard place; I'll sell it to you at the price you quoted."