It is interesting how sometimes just a turn of phrase or slightly different view of something can change your understanding of it. You may not know anything new about the subject, but how your understanding of how the different parts of what you know work together changes.
As I remark in my book, blocking/ parrying actions can be described as resembling a diamond shape. A parry with either limb is either inward/ high, outward/ high, inward/ low or outward/ low. Karate is one of the exceptions to this model since the direction of the defensive hand moves are better visualized as being either high, low, outward or inward.
Recently reading a book on knife techniques where the author comments that any attack can only come from one of four possible quadrants, which correspond to inward/ high, outward/ high, inward/ low or outward/ low. What was novel to me was that he visualized these four quadrants by imagining a vertical cross shape, centered on the elbow. I suspect this model may originate from some school of the Filipino Martial Arts. For any attack from a particular quadrant there was only one basic defensive move. A high attack on the inside would be met by an inward cut with the point of the blade up, a low attack on the inside by an inward cut with the point down. Attacks from the outside of the elbow would be met with outward cuts, point up or down depending on if the attack was high (above the elbow) or low.
What works with the knife often works with empty hand techniques. Let us consider attacks from the four quadrants (relative to the elbow) and how we are likely to deflect them.
High Outside Attack: Probably the most likely response is the thrusting or outward swinging action we see in the outward karate parry. The same fundamental action can be seen in boxing and many other fighting styles.
Low Outside Attack. Most responses are variations of the Karate low parry. Drop your hand and let it swing to your outside, turning your hips outward. With a knife you would make this as a cut, with unarmed you would deflect with the hand or forearm.
High Inside Attack. This body motion used to counter this attack has a number of variations. First we have the inward parry of Karate, using the forearm in an approximately vertical position. This is quite a strong technique and covers a good area. Secondly, we can use the hand to make contact with the attacking limb and strike or push it away. This has a little more reach than the Karate inward parry but is less forgiving when it comes to accuracy. Thirdly, we can punch or thrust past the attacking limb and make contact with our forearm. Which variant you use will depend on the actual attack and what you are most adroit at using.
Low Inside Attack. This is an interesting area, since in the Karate model all low attacks tend to be dealt with by sweeping them to the outside. Most attacks that come in under the elbow can be dealt with this way. It is also possible to deflect inward. With a knife this would be an inward cut with the point downward. The thumb will be low and the palm upward to hold the knife edge in the correct orientation. The little finger side of the arm would be the “leading edge”. With the empty hand the low inward parry is more likely to be made with the palm towards you and the thumb side the “leading edge”. Contact area is likely to be the thumb side of the hand or forearm or the outside of the forearm. Actual technique may also vary with the height of the low attack. The above assumes a relatively straight arm is used. For a higher low attack a variation resembling the Bong Sau (wing arm) of Wing Chun might be useful. Here the contact area will be the ulna edge of the forearm or its outer/ upper surface.
Can you defend with just four basic motions? Possibly.
This illustration from the book shows eight defensive moves from JKD and if you look carefully you see each covers one quadrant for either the forward or rare arm. Are the techniques the simplest/most efficient/most fumble-free for a quadrant? In the book I discuss how the P’eng Hinge stance can be used to deflect the majority of attacks with just and outward or inward rotation. There are parallels to this in staff or bayonet fighting too. But there will also situations when an additional technique or variation can do things more effectively. Attacks to the low inside quadrant are a good example of this.