Using slow rotation to control the interfaces in horizontal zone-melting (HZM) is a widely adopted method for organic materials, i.e., the Pfann-Miller technique. A short and nearly axisymmetric zone can be achieved easily even with a rotation rate of only 1 or 2 rpm. However, the melt convection might not be the crucial mechanism for such a control. Through flow and interface visualization during NZM of succinonitrile, the role of the slow rotation was investigated. Three-dimensional numerical simulation was further performed to illustrate the key mechanisms for determining the zone shape. Besides the good agreement between the simulation and the experiments, we found that the heat of fusion released or absorbed during the rotational regrowth and remelting played a crucial role, and it could not be ignored in the simulation.