Rotation of a helical magnetic limiter in the poloidal and toroidal directions is shown to be a good method of avoiding the non-uniform wall loading that is obtained when a static local helical field is applied. Mechanisms of the static and dynamic modification of the plasma potential at the tokamak edge are discussed in terms of magnetic field structure, ambipolarity, electron transport along the stochastic magnetic fields and cross-field drift motion of plasma particles. The magnetic structure is studied by Poincaré mapping of field lines traced numerically, using a code that accounts for the exact configuration of local helical coils.