The 45 degree constraint is the default. Leave as is.
To use the Knife you simply left click and drag across the full surface.
Holding down the Shift key will lock the knife to make straight lines, or at the angle specified in Constrain Angle.

Measure out some 'windows' on the right face. This view is zoomed in to see the minor grid.
I'm making 3, two pane, vertical windows. The 3 extra horizontal lines you see will be used to make window frames.
The Knife Adds\Creates New Polygons - it does not cut!
It is manual surface-line division and a great way to make low weight features without using subdivide.
Subdivide Quadruples the polygons but the Knife gives you much more control especially on simple, non-organic features.