A word of caution. It has been known for the system to sense the trigger for power during the landing flare as the speed reduces, if this takes place before the (27' I think?) 'Retard' point. Can be a bit disconcerting. I have watched it happen to the other pilot and it doesn't half mess up the landing!
I think it works on 1.3xVstall, as sensed by the alpha vane. There are points in the approach profile (737) where (if ACCURATE +5kt speeds are flown - what!?) the trigger point is actually ABOVE Vref+5. This occurs at a few flap 30 weights and more flap 40s. Use with caution.