A cross-section consists of a node set and one or more element sets. A node set alone does not constitute a cross-section nor does an element set alone constitute a cross-section. A correctly defined cross-section contains nodes which define the cutting surface and deformable elements to ONE SIDE of and touching that cutting surface. Elements of rigid bodies do not contribute to cross-sections, that is, only deformable elements should be included in cross-sections.
When the PLANE option is used to define a cross-section (*DATABASE_CROSS_SECTION_PLANE), LS-DYNA chooses the nodes and elements making up the cross-section. The nodes and elements chosen for that cross-section are reported in the D3HSP file under the heading "interface definition".
*DADABASE_SECFORC determines the output interval for the cross-section forces and moments. The forces and moments are output in the global coordinate system, or, optionally, in an updated local coordinate system attached to a rigid body or accelerometer.
As an alternative to defining a cross-section in the LS-DYNA input, in which the cross-section follows a particular set of nodes and elements, you can use the SPLANE option in LS-PREPOST to gather forces/moments at a cross-section which is fixed in space. The sequence of LS-PREPOST operations is SPLANE > (define location of cross-section) > CUT > FORCE > (select force or moment component) > PLOT. When plotting forces via SPLANE in LS-PREPOST, the cutting plane is fixed in space and does NOT follow the deforming material. In other words, the set of elements, which are cut can change as the model deforms (the FixM option appears to have no effect). Also, force contributions come only from those parts, which are displayed.