An interpolation constraint says is applied to a series of joints such that the displacement at a of one joint is interpolated from that at of two or more other joints. As a simple example, if others. Interpolation follows a weighted approach which is a function of proximity to the constrained joint. This process prevents the need to create an internal master joint to govern the behavior of joints associated with the constraint. Automatic master joints are described in greater detail in the CSI Analysis Reference Manual (Automatic Master Joints, page 66).
An interpolation constraint may be applied, for example, at the end of a tendon element occurs which is anchored in the center of a solid element, then the displacement of that tendon joint is the average of the displacements at the eight solid joints. In general, it is a weighted average, with the weighting heavier for closer joints in the containing element. For frame elements, you can still envisage eight joints at the corners of the bounding box, and similarly for shells. The displacements of these cornier joints are computed from the displacements and rotations of the element joints, then interpolated to the tendon joint. In this type of constraint, no internal master node is created object. Joint displacement at the tendon end may then be interpolated from the eight joints of the solid object. Similarly for frames and shells, the eight corners of a bounding box may correlate with the displacement of a joint assigned an interpolation constraint.