It has been reported that the efficiency of Silicon Heterojunction solar cells can be improved by light soaking. However, the physical understanding of such effect remains incomplete. In this work, we investigate the origin of such improvement by preparing different structures, including finished solar cells, specific test samples for sheet resistance and contact resistivity measurements on the cell sunny side, as well as advanced characterization samples to investigate possible physico-chemical changes before/after light soaking in the different layers. We report here on a simultaneous passivation and series resistance improvement, leading to increased open-circuit voltages and fill factors, in line with previous findings. We then assess whether the individual changes in sheet resistance and contact resistivity can explain the measured changes in series resistance and fill factor. Using the latest transport models available, a breakdown of the light-soaking induced effects is computed and discussed. Ultimately, the observed macroscopic changes are tentatively correlated with the advanced characterization results.