The manual says :
There is a tool that comes up in blender 291 under the search that is called “Weld Edges into Faces”
its defined like this:
A tool to split selected faces by loose wire edges. This can be used in a similar way to the Knife tool, but the edges are manually setup first.
What is a WIRE edge ? ? and what would make it be loose ?
A wire edge is an edge not adjacent to any faces. I.e. if you select a single vertex and extrude it, you get a wire edge. Or, if you select two vertices and hit F, you make a wire edge. Which is a common beginner mistake when wanting to cut a new edge in an existing face.
…now I’m wracking my brain trying to remember how to Split Faces* using existing points on their periphery…
Man, aging memory sucks. *Thank the devs for the Knife tool.
Connect Vertex Path (default keymap J).
A little video that explains what J does compared to F