Dynamic Relaxation (DR) is a simple but effective technique to initialize structures to pre-loads prior to any primary load application. LS-DYNA has supported DR for several decades and is used by several users. Recently, in an attempt to improve convergence (Explicit DR or Implicit DR), there was a need to HOLD certain portion of a structure to reduce undesired movement. The problem with enforcing such constraints using BOUNDARY_SPC was they continue to be present in TRANSIENT phase as well which may not be desired.
One method to overcome this was to use BOUNDARY_PRESCRIBED_MOTION keyword (one for each DOF) and specify a zero displacement as a function of DR time and set SIDR=1 in the define curve. However, recent update to R5 enables uses to use BOUNDARY_SPC with BIRTH_DEATH option with DEATH eq 0.0 which causes the spc constraints to be honored in DR phase but they die as soon as LS-DYNA enters TRANSIENT phase.
Please refer the latest LS-DYNA manual under BOUNDARY_SPC for more information.