Before revision:  Heparin can mask both the S1/S2 basic motif, thereby inhibiting furin cleavage and the formation of the prefusion conformation, and the basic residues of the receptor binding domain (RBD), thus acting on the hinge region responsible for the motion of the RBD between inactive closed and active open conformations of the spike.
After revision:  Heparin can mask both the S1/S2 site, which is unique specific to SAR-CoV-2 and is important for furin-mediated spike cleavage and the formation of the prefusion conformation, and the basic residues of the receptor binding domain (RBD), thus acting on the hinge region responsible for the motion of the RBD between inactive closed and active open conformations of the spike.