The purpose of a hard block is to “attack” and hurt the opponent's attacking limb and so protect yourself. In so doing the opponent's attack is forcefully redirected. For this purpose the blocking tools chosen have hard surfaces that are conditioned to make them harder and more resilient against pain. Although practically all ITF Taekwon-Do techniques have some circular motion as part of their preparatory movement (what's referred to in the Training Secrets as “backward motion”), hard blocks usually accelerate towards the target in a straight line. Typical examples include most forearm blocks, knife hand blocks, and the front fore fist pressing block. This latter block, although technically a checking technique, clearly illustrates the nature of a hard block, which is to hurt the opponent as the front fore fist pressing block is basically a punch directed at your opponent's kicking foot (instep).
Soft blocks, on the other hand, tend to reach the target in a curved line using a circular motion principle, or when it reaches the target straight on the function is not as a strike, as the case is with hard blocks, but as a push. The purpose of soft blocks are to deflect an attack by redirecting the force of the attack, or to unbalance the opponent using some kind of pushing motion. Unlike hard blocks that put emphasis on hurting the opponent's attacking tool, soft blocks put emphasis on redirecting the force of the attack and / or breaking the opponent's equilibrium. The hooking block is probably the most easily recognised soft block, but other soft blocks include the upward block (“upward” is not to be confused with “rising”), palm downward blocks, palm pressing blocks, pushing blocks, the grasping and luring blocks, scooping blocks, circular motion guarding blocks and some checking blocks.
Body shifting is the umbrella term for another group of defensive techniques and include dodging, foot shifting, different types of stepping, sliding, turning and jumping, as well as body dropping and foot lifting. The purpose of most of these body shifting is to “avoid colliding with an [attacking] opponent.” Dodging requires some type of foot shifting; that is, moving one's feet away from an attack and so shifting your body out of harms way, but also placing you in a more advantaged position for counter-attcking. You can move either one foot to shift your body positioning, or both feet. Avoiding an opponent's advance can also be achieved through stepping, sliding, turning, jumping or a combination of these, which will cover a greater distance than only foot shifting can achieve. With regards to stepping, turning, and step-turning the ITF Encyclopaedia identifies quite a number of possible combinations and are often used as part of other block techniques and can even include standing grappling style techniques like those often associated with Aikido, which involve blending with your opponent's motions. Then there are also partial body shifting techniques that does not require the whole body to change location, like foot lifting, which is what you will do when someone tries to sweep one of your feet. Body dropping is probably better known as “ducking” when avoiding a very powerful high attack, for instance a baseball bat swinging at your head. A strong attack aimed at the legs could be avoided by a high jump, with the legs tucked in as illustrated in the photo. Other partial body shifting includes “bobbing and weaving” which is often employed by boxers, but also used in Taekwon-Do.
Finally, guards, also known as guarding postures, are a more passive type of defensive technique used in Taekwon-Do. A guard is basically a posture, a method of standing and placement of your limbs in such a way that if your opponent's attack should reach you before you could avoid or block it, your body positioning and placement of your limbs function as a buffer that cushions the blow. A guard generally closes off many of your own vital spots, acting as a fence between you and your opponent. Some guards also put you in a preparatory position from where it is easy to zap out your own attacks; these positions are often referred to as "chambers." The accompanying photo shows a chamber position, known as the bending ready stance, from where one can easily launch a side-piercing kick or jab, if an opponent should venture within range.
It is clear that there are a variety of defensive techniques in Taekwon-Do. Each type is worth exploring and provides useful methods of defence. While it is good to consider each type—hard blocks, soft blocks, body shifting and guards—thoughtfully, it is very important to remember that they are all integrated and often work best in combinations rather than in isolation. Nonetheless, hard blocks tend to get the most attention in most ITF dojang, therefore I would advice you to spend some extra time on the other types of defensive techniques so you can become familiar with their strategic values as well.
Images from Sonkal.