I've always assumed that too aggressive carving beneath a branch could compromise it so, if there's any question, I always start small and build on the work over time to allow the tree to develop new pathways for sap flow. Carve with the grain and don't be afraid to carve deeply if it's appropriate... fat trunks should have deep hollows, imo. I always start in one or a few places on the trunk(s) where I know I want a carved feature, then build on the work over several sessions over a season or three, studying what's been done as I go. I'm looking forward to seeing your work!