I'm sorry, perhaps geometry isn't my thing, but I'm not seeing how this slice makes the desired rectangle, could someone post the compleated 4x6 grid as made bythis slice?

I'm with Incognitum, the 4x6 grid shown in the solution isn't really a rectangle, as far as I understood the definition of a rectangle anyway. Doesn't a rectangle have to have 4 right angles (and thus 4 sides)? The solution only has two right angle and actually has 6 sides.