The reciprocal arm of a removable partial denture is designed to provide lateral stability by applying a counteractive force to the tooth when the retentive arm engages undercuts during insertion or removal. This prevents unwanted tooth movement or tipping caused by the retentive arm. Contacting the tooth only when the retentive arm is activated ensures minimal stress on the abutment tooth during function and avoids continuous pressure that could cause trauma or discomfort. Hence, the reciprocal arm should engage only during removal or insertion, not at all times or never.

Reference: Removable Partial Dentures, Carr and Brown, 13th Edition.