TAD Docs

This action would remove the current edge, and in its place it would make 3 edges; of which the middle one would be the same as the one that was removed; except that this new edge is created at the location of the architect. The other two edges that are created would be orthogonal to the central edge

In short, this creates a rectangular shape, with one edge of the rectangle missing (which is where the first edge used to be)

You can always locate the architect accurately using the headsup command line, before invoking this action.