How fast can small electromagnet change it's power?
This is an interesting idea, do you want to make this threshold variable? If not I would have just gone for a mechanical switch that gives the same effect. Electromagnets/Inductors tend to not like changes in current, so the change won't be instant, it will probably so fast that you won't be able to feel it though. You could speed it up somewhat by placing a high speed diode across the terminals of the magnet reverse biased, this helps collapse the magnetic field once the current is disconnected.