A sitting target is easy to hit; a moving target is difficult to hit. For an opponent to strike you with damaging force, the opponent must be set. Movement must stop for at least a moment so force may be generated from the floor, through the legs, and out an arm or leg to the target. If the opponent sees an opening and sets for the attack, if you are constantly moving, the opening will have closed or moved before the attack is fired. The attacker may try to "lead" the target and fire to where he or she thinks the target will be when the attack reaches it, but if you move erratically, there is no way for the opponent to predict where the target will be. This is why boxers move continually and why most of the attacking punches miss their target. To make up for this, boxers throw and array of punches in all directions, hoping one will make contact.
Therefore, the primary way to avoid an attack is to move; then blocking is unneeded or needed minimally. Stay on balls of feet and keep the feet moving (dancing). Erratically, but purposely, weave the body from the ankles to the neck. Erratically, but purposely, bob and weave the head using the neck and upper body. Erratically, but purposely, move the entire body in circles around the opponent. When all this is done, the opponent cannot set, so, even if attacks strike a target, they will be weak.
Blocks are used as a last resort when all else has failed, such as avoidance (sidestepping or ducking) or leaving. Even then, the block may fail. When a block fails, you get hit. Even if a block works, your blocking arm or leg may be injured. Blocks are violent—force is meeting force. Blocks require a lot of energy, they stiffen the body, and they cause mental as well as physical stress.