Constraints should really be called restraints but the wrong use of the name has its history in Rosetta. A constraint in Rosetta is implemented as a biasing potential, often harmonic, that causes an energy penalty if a certain parameter strays far from its set value. If the total energy is optimized during a structural simulation a constraint thus can increase the number of final structures for which the respective parameter is close to the set value of the constraint. If one really wants to constrain a certain degree of freedom, i.e., that a certain length should be exactly the specified value, or that a torsion is not moved, one enforces such things usually by carefully selecting the move-set (such that a particular internal degree of freedom is not moved), and by changing the kinematics such that the length/angle or torsion of interest become an internal degree of freedom (see Constraints in Rosetta).