Hi,
I just updated the tutorials to explain how we implement the projection, see
ngsolve.org/docu/nightly/i-tutorials/unit-2.8-DG/DG.html
,
section Projected Jumps.
Try also help(FacetFESpace).
The same works for H(div)-conforming HDG, with a TangentialFacetFESpace.
The theory behind is in Lehrenfeld-Schöberl: High order exactly divergence-free Hybrid Discontinuous Galerkin Methods for unsteady incompressible flows, CMAME, Vol 307, p 339-361, 2016.
best, Joachim