An object in motion tends to remain in motion until acted upon by another force. An attack must be stopped, deflected, or avoided. The attack may come in either a straight line or an arc. Stopping the attack by meeting it head-on may be painful and you may not always be able to get out of its way. Therefore, the best strategy is to deflect the attack. A small force that cannot stop a large force may easily deflect the large force. Deflections should be used in a circular motion.
An unskilled person will hurl their body, more or less uncontrolled in one direction, with their strike. They are functioning more or less like a falling rock. A skilled martial artist who understands the principles energy will not move in this manner. If one part of their body goes forward, another goes backward (Yin/Yang) and the forces are balanced. Therefore, pulling this person in the direction of their strike is difficult, because their inertia is balanced and not focused in one direction.
For greater striking force, you should strike the opponent on the same line as his/her inertia. If a strong part of your body is striking a weaker part of the opponent, there is no problem. However, if your weapon is not strong enough, relative to the target, this method will result in injury to yourself. If you meet force on a line perpendicular to it, your small force can deflect a larger force.