Magician:
To win a game you don't have to pedal at all, just take a PWM 50% duty from arduino, 490 PPM!
I think you miss the point here... If I don't pedal, why use the training bike at all? Fun + exercise = win!
Why placing two/three magnets on the wheel would require precise phasing? It's not a rocket technology
Two magnets won't cut it, for the reasons I have stated above. The crankwheel is twice as big as the sprocket, so on the average there are two turns per second. With eight magnets the response for braking (change of direction) would be almost 1/8 s - passable, but not great...