Buffer in construction management means a cushion that protects the planned project schedule from uncertainty. Traditional buffer management, however, often fails to perform its purpose, hence reducing efficiency in schedule management. Such failure has been mainly attributed to the fact that traditional buffering is based on experience rather than being made in a systemic manner. For this reason, in this paper, we investigate the inefficiencies of the traditional buffer, especially focusing on the manager's assignments. In this study, in-depth interviews were conducted on sixteen schedule experts in twelve companies in Korea to explore their scheduling practice, and interesting facts were found. By analyzing the interview results, this research identifies three fundamental problems of the traditional buffering, based on which, Assignment Buffer Control (ABC) is proposed. ABC improves the problem by metamorphosing the buffer and incorporating manager's assignment direction to the schedule.