The way the steady flow computations proceed is from downstream to upstream for subcritical flow and from upstream to downstream for supercrical flow. For possible mixed flow regimes, both profiles are computed. The basics of this type of computation are given in http://pubs.usgs.gov/twri/twri3-a15/pdf/TWRI_3-A15.pdf.
I studied your document, but this is only steady flow (sub, super-critical and mixed).
What direction does hec-ras compute, when i solve unsteady flow (gradually or rapidly varied flow)? I was looking also other documents, but i found nothing
Sorry, I interpreted your question to refer to steady flow. The step-backwater procedure is only for steady flow conditions. For full unsteady flow (St. Venant Eqs.) the unknowns (stage and discharge) are solved simultaneously at each computational point, stepping forward in time. The actual computations involve solving a non-linear matrix by iteration at each time step. So, there is not an upstream/downstream computation for unsteady flow. The formation of the matrix is described in detail in the HEC-UNET manual.
Michael