but what's the exact science behind it
The sensor output (an impulse stream) lets the PM measure the maximum and minimum flywheel speeds and the times when these occur.
The loss of speed during recovery and the flywheel inertia give the Work done on the atmosphere by the fan during recovery between strokes.
This Work is then extrapolated to the full stroke and this, divided by overall stroke time, gives the rate of work (Power) in Watts, shown just after the pull ends.
This system made the erg (with a flywheel) perhaps the only exercise device that measures Power, simply and accurately.
SPM derives from 60/stroke time while pace (or speed) derives from a cube law where W = k V³. k is set to give the speed of a coxless four. Pace is 500/V.
08-1940, 179cm, 83kg.