Here are some infos, which I found out for myself.
Snapping-functionality has at least two preconditions:
1) The GraphicsLayer resp. FeatureLayer which should be snapped to must have the property IsHitTestVisible="True".
2) You have to know the key, which activates the snapmode. The Default is Left-Ctrl. Use Editor.SetSnapKey to change this.
Snapping is also possible without editing a FeatureLayer, eg. drawing geometries on a draw-object while snapping on GraphicsLayer and FeatuerLayer.