The reciprocal arm of a clasp assembly in removable partial dentures must be always passive to provide a stable counterforce against the retentive clasp arm during insertion and removal. This prevents tooth movement or trauma by stabilizing the abutment tooth without engaging undercuts or causing excessive pressure. Active force from the reciprocal arm could lead to tooth mobility or periodontal damage. Therefore, ensuring passive contact is critical for maintaining both tooth integrity and prosthesis function.

Reference: Contemporary Fixed Prosthodontics, Rosenstiel, Edition 5.