Double-clicking will open a Node Editor:
The Nodal Displacement destination node has a Mode setting to control if the value is used to displace (add) or set the vertex position. It also has a setting to control if the Deformed Coordinates in texture nodes are in object coordinates or in world coordinates. Double click to access these options.
The Input node also has a Current Position output providing the current deformed vertex position in object coordinates (the existing Object Position is the undeformed position, and World Position is the deformed position in world coordinates). There is also a Point Index output giving the point index of the vertex.
The nodes for Nodal Deformers are described here.