Knife projection is a non-interactive tool where you can use objects to cookie-cut into the mesh rather than hand drawing the line. This works by using the outlines of other selected objects in Edit Mode to cut the mesh along the view axis, resulting geometry inside the cutters outline will be selected. Outlines can be a wireframe or boundary edges (i.e. the unconnected edges of a mesh).
The primitives, being manifold objects, do not have wireframe or boundary edges. In the case of the cube, deleting the top face will result in cutting edges.
Select Non-Manifold (Wire, Boundaries) will highlight the cutting edges of mesh objects.
To use Knife Project, first while in Object Mode, select the cutting object then add to that selection with Shift-LMB the object to be cut. Now, enter Edit Mode and choose.
3D Viewport Alignment to adjust the projection axis.
- Cut Through
Projects the cut through the entire mesh.