When someone strikes at your leg, pull the front foot back, or step backwards and deliver a downward cut to his head as drawn here. Although with a sword in two hands you should not strike to the knee or below as there is too much danger to the one who strikes, because whoever strikes to the leg has no cover. If one has fallen to the ground, then you may well attack the leg, but otherwise, do not do it standing sword against sword.
With a sword in two hands, you should not strike at the knee or below. Here, the 6th scholar of the 2nd master gives a clear demonstration as to why not. It all comes down to geometry.
Imagine a circle whose centre is at the shoulder and whose radius extends along the line of the arm and sword. The circumference of the circle is at the swords tip. Where the radius of the circle is horizontal, it will reach the opponents head. This is shown by the scholar. Where the radius of the circle reaches for the opponents legs, it will fall short, as you can see with the player.
In practice, this makes for quite a simple technique. A cut to the leg is a relatively large movement, and you should be able to see it coming. Do not bother trying to cover it, but simply slide your front foot back out of range. Try to keep your shoulders in more or less the same place. As you step out of the way of your opponents cut, make a cut of your own to their head.
Your strike will be like a short snap driven by your arms rather than your hips. You should finish in Posta Longa. It is not a powerful cut, but instead relies on accuracy and a complete absence of cover from your opponent. Strike right into the centre of your opponents forehead.