Transient interplanetary shocks distort the interplanetary magnetic field as they propagate through the interplanetary medium. This distortion involves a deformation of the magnetic field lines and a modification of the magnetic field strength. Both factors have influence on the propagation of energetic particles. Magnetic trap regions, for instance, may appear around local minima of the magnetic field in the downstream region. We briefly discuss the problems that appear when modeling the transport of particles there, and we present a simplified model to describe the influence of the focusing effect on particle propagation. ©1999 American Institute of Physics.