Well, when I said they 'jiggle', I was exaggerating slightly. You, the USER, do not have full control over them as the state of the eStop affects them, and they change state during power-up. This is as A_Camera says.
The big problem with them in my real situation is that they are not compatible with the Gecko 203V servo drives. The Enable input on the Gecko is also the Fault output, so you can't just push it Hi or Lo. You have to sense it as well as driving it. In most cases today it may be best to just ignore them imho.