In hydrocyclone separation, the fine end of the separation curve T(d) frequently becomes a line parallel to the abscissa of the diagram at T > 0, which is caused by the turbulent flow character of the hydrocyclone flow. But in the classification of feeds < 100 mum in small-diameter cyclones, several authors have found curve shapes that show a minimum in the range of about 10 mum or below and a rising of the curve again towards the finest end. This phenomenon is called the fishhook effect. Toward this end. a model is presented based on the flow forces having an effect on fine particles in the Zones of the shear gradients (boundary layers) around settling coarser particles. These forces have an attractive component so that "swarms" enriched by fine particles can form around coarser ones. As a first approximation, the Reynolds range of the settling coarse particles where swarm formation can occur is suggested as Re-p approximate to 0.5 to 25. The upper limit is given by beginning separation of the flow around the coarse particles, the lower limit by the requirement of inertia effects. Such a swarm is a dynamic association, which is in continuous exchange with with its vicinity by particle motions (directed and diffusive motions). From the influence of dynamic lift force F-D, a reduction of the mobility of the fine particles results in the swarms; consequently, an increase of their retention time as well as concentration is observed. The presented approach of swarm formation is verified by extensive test results using quartzite of different size distributions as materials. Finally, on the basis of the new approach an explanation can be given why this effect could only be found in almost every case at the classification in small-diameter hydrocyclones.