here's an extreme example http://www.youtube.com/watch?v=8PiGVQG7GO8 and yet the smash speed was still around 250km/h or so...
Focus on your footwork and get your body to an ideal position before striking the shuttle, everything else is secondary,
wow that was pure wrist. just sticked his arm up and snapped with his wrist. that certainly takes a lot of forearm strength.
In the video, the coach was just showing him the isolated technique of smashing with more wrist speed and power. In an outright power smash, that isolated technique of wrist would be incorporated into the whole smash movement (ie. rotations/movements of the hips + shoulder + elbow + wrist + fingers). Depending on what type of smash you are trying to accomplish you would minimize/reduce this combination of movements. Power smash --> use all, weak smash --> maybe just eblow + wrist + fingers