Sensor in pedal assist pedicab

I don't have the exact knowledge of the system you are talking about, but most probably, there is used a Hall-effect based sensor. When there are the magnet field changes around this sensor, it responds on thus changes by means of current generation, that can be used forward with controlling purpose.

Try to find more Hall-effect on http://en.wikipedia.org/wiki/Hall_effect

With best regards

Good question. I can only guess. Ordinarily you would want to measure torque, because you only need help when pedaling torque is high. But how would the relationship between the five magnets and the sensor change with torque. (It would clearly change with speed, of course.) If there is some compliance built into the bottom bracket, then the distance from magnet to sensor would change, and that would change the magnetic flux -- but it would be a very small change, particularly because the bottom bracket area should be very stiff for good power transfer. But if the gap is small, and the sensor/magnet relationship is optimal, then the change of gap (and thus flux) that would occur with frame flex could be used as a control input.

There was an earlier thread on a related issue: http://cr4.globalspec.com/thread/5979/Measuring-instantaneous-power-on-bicycles

You could probably find out more with a patent search.