They are pizeoresistive elements that change their resistance values according to how much mechanical stress they are under. Thus, by measuring how much resistance the element has, you can calculate the force it is being exposed to...and by extension the acceleration (F=ma). There are 3 axis in which these elements are aligned, so you can, with some fancy math, calculate the 3-d force vector the device as a whole is experiencing at a given moment. With the proper axis alignment and equations, you can "simulate" a gyroscope's ability to measure angle changes in most cases.