I would call your instructor's comment a "rule of thumb" and a "brainteaser" of a sort. Don't get too hung up on it. Nevertheless, there is merit in what is said, and it can apply to strikes. But this can only happen at a very advanced level.
When you learn sanchin, you have it ingrained in you that your hand must be hard, and yet your striking arm is relaxed (in the flexor muscles) as you extend it. This hardness of the hand must stay for some techniques such as a shoken. It is like the hard, tempered edge of a sword that is only molecules thick over the folded layers of soft steel. But the extensor muscles in the arm do not necessarily need to be the origin of the power of the strike.
Recently I read of a case of a person who was inadvertently hit in the neck by the stray heel of a person who was being thrown. The uke was obviously relaxing and absorbing the energy of the throw so as to achieve a gentle landing. But the energy of the throw went through this person's body, and was expressed as centrifugal force in the flailing, spinning legs. When the foot hit this person's neck, it damaged her carotid artery; she later needed emergency surgery to repair the damage. This is a very, very rare and serious injury - the kind you see in an automobile accident.
Now in this case, the heel of the foot was obviously naturally hard. If you have practiced sanchin long enough, your hand will be able to remain hard (in a shoken perhaps) while the rest of your body relaxes. Once you achieve this state, the stage is set for your body to have energy flow through it, and have it delivered by the tempered tip of the weapon on the end of your hand. An extremely skilled practitioner can take energy directed toward them (as did the uke in the previous example) and redirect it through the body and out again. And when you are *really* advanced, you can learn to take advantage of natural body responses like the stretch reflex to actually enhance the energy flowing through your body. But the second you stiffen up in the wrong place, you dampen the energy wave.
It isn't easy, but the possibilities are there. It seems and feels like magic when it is done (and that - in my opinion - is why some people believe in chi).
Hope I helped.
[This message has been edited by Bill Glasheen (edited 10-23-98).]