"Shear modification" is a strong modification of rheological properties which affects mainly long-chain branched polymers like LDPE. The aim of this work is to explain this effect using recent advances in molecular dynamics and especially the pom-pom model which was designed for branched polymers. The original model was slightly modified in order to take into account the change in molecular topology related to the branch point withdrawal mechanism without introducing any further assumptions. A theoretical approach shows that when arm retraction is significant, the whole relaxation time scale is modified inducing a very different rheological behavior in shear and in elongation. Results are in qualitative agreement with reported experimental results, but a quantitative description requires some refinement in the molecular dynamics theory and would necessitate the use of well calibrated pom-pom-shaped polymers for quantitative experimental evidence.