This occurs on uneven terrain.
For example, in a plant with east-to-west slopes and a homogeneous pitch, shadows can differ significantly between morning and afternoon. To minimize shading, it's common to set a lower (fictitious) backtracking pitch than the real one when the tracker is facing uphill (either east or west, depending on the case).
I know I could simulate this using two separate PVsyst simulations and then combine the results, but of course, it would be better if this could be configured directly in PVsyst.
Thanks!