Onto Diagonals:
Despite anything else you've seen in this training, diagonals will be the attack you'll use most with your sword, unless you really adopt a fencing fighting style. The reason for this is simple: we're human.
We don't move in perfectly straight lines, nor do we attack in them. They'll usually have some kind of angle to them; they won't ride perfectly on the x and y axises.
Diagonals are hybrids of both vertical and horizontal strikes, being offset from both axis. However, being, in a more practical sense, the only type of slash you'll ever make, they can block any slash as well, depending on the angle.
A nearly horizontal slash would be incredibly difficult to block with another horizontal slash, but a more vertical slash would have no problem.
That's the main premise of diagonals: angles. Depending on the angle of your blow and your opponents, you'll either block or trade blows.
While more of a technicality than something you'll see in the RP, it does raise important points.
Horizontals and Verticals stop each other by making pluses ( + ), while diagonals do so by making "X"'s. Like I said, the angle of the blows will always differ - we're human - but it address how strikes interact.
Questions? Comments?