A strained voice is what you hear when you contract your abdomen and diaphragm, while closing your larynx. This combination of actions forces a small current of air through your throat, vibrating your vocal cords in a manner markedly different from the easy, unencumbered vibration that characterizes your normal voice.
Strained voices can be heard naturally during any kind of exertion that requires abdominal tension, such as when a constipated person is on the toilet, and must create pressure in the abdomen to aid in the process of moving his or her bowels.