Knowing what power is, the question becomes: how do we measure it? Power meters measure the power output (often simply referenced as PO) of a cyclist. They use strain gauges to measure the strain (or in the case of a cyclist, the torque, which is another way to express force on an object rotating about an axis). These training tools became commercially available in the late ‘80s when strain gauges were placed in either the hub of the rear wheel, or on the bottom bracket of the frame itself (see images below).
I look forward to a similar review covering the main brands of stationary bike, including Wattbike http://wattbike.com/uk/ (though I concede that this is quite a different training tool to all the other stationary bikes)