No current at all. Imagine a balanced metal arm pivoted in centre and a coil at each end with some remanent magnetism. A current pulse through either coil is sufficient to overcome the small static magnetic pull and trip the bistable to the other condition. The Danfoss WP75H uses a much larger Gruner 703H to switch up to 25A for an immersion heater.
This article has a short explanation and some driving circuits:-