When lifting something, the further the arms are from the body the weaker they are, which is why it is difficult to lift a heavy object with out-stretched arms. This is due to the increased torque required by the larger radius of rotation. However, this does not translate to a linear punch. The more extended the arm in a punch, the more difficult it is to collapse the elbow, and the more force is be transferred into the target, which why people push heavy objects with their arms fully extended.
If the intent of your punch is to push the opponent backward, keep the elbow bent and the arm close to the body to allow you to apply more pushing force, as is the one-inch punch. If the intent of your punch is to damage the opponent, whether or not the opponent mores backward not , then punch with an extended arm.
Watch people while they are kicking a body shield held by another person. If the kicking foot makes contact with the shield with a large bend in the knee, the result will be a pushing kick. The foot will penetrate into the shield until the absorption limit of the shield, make a connection with the holder, and stay in contact with the holder until the holder is knocked backward. It will be a sudden acceleration backward but the holder will feel no ill effects.
However, if the kicking foot makes contact with the shield with a slight bend in the knee, the result will be a snapping, thrusting kick. The foot will penetrate into the shield until the absorption limit of the shield and make a slight penetration into the holder. The shield holder will only move slightly; however, the holder will experience a quick, powerful jolt that will shake the holder from head to toe. If the kick is done properly, the holder will feel as though his or her eyes are rattling in their sockets, that their tooth fillings have been shaken out, or that, for a brief moment, they were unconscious; it will not be a pleasant experience.