In low Reynolds number flow conditions, an isolated mixing region (IMR) consisting of two troidal regions is formed in a stirred vessel, and furthermore one or several string IMRs are occasionally observed surrounding the troidal IMR. The string IMR has a closed spiral curve and rotates around the troidal IMR. The winding number and the mean rotating velocity of the string IMR are generally different from those of the fluid particle orbits. In this study, we explain the mechanism of the string IMR formation where the rotating motion of fluid particles is synchronized with the perturbation of the propagative wave motion caused by the passage of paddle blades. We proposed equations that relate the string number, winding numbers and mean rotational velocities of the string IMR to the rotational speed of fluid particles and the number and rotational speed of paddle blades. These relations were validated by comparison of the calculated results obtained by the simplified flow model with experimental results.