Rather than we old duffers discussing proper technique, perhaps we should watch the best to see how it is done. Here is a video link of Olympic Champion
Lisa Carrington showcasing her technique. Admittedly, this video shows the high angle racing technique of a world class sprinter, but what you see her demonstrate applies to a powerful, efficient touring stroke as well; just a slower cadence and a little more relaxed. The base principles are the same.
The stroke is a whole body thing, beginning with the legs, helping to drive the torso rotation. Power is provided with the legs, core and upper back muscles. The arms remain almost straight and strong, but as was mentioned above, your straight leading arm provides a push powered primarily by your lattisimus dorsi muscles, while your trailing arm provides a slight draw, also powered by your back and core rotation.
The latissimus dorsi are the largest muscles in the upper body and hence won't tire as easily, as opposed to over using your arms. I would think that if you use your arms, as in bending at the elbow to pull or push the paddle, you would simply replace powerful core energy with lesser arm energy. You need strong straight arms to efficiently use the power provided by your back, legs and torso. It would seem to me that you need a strong frame to avoid sloppiness and to push the boat most efficiently.
When I learned stoke technique, I was taught a low angle endurance stroke, but again, the same principles apply. In a similar vein to kayak Jim, I was told to pretend I had a sign pinned between my shoulders and to show it to the shore on both sides of the boat. If you watch Ms. Carrington from the front, you will note that her leading hand crosses the centre line of the kayak on every stroke. That is what I was taught as well. I push across with my lead arm powered largely by the upper back muscles, torso rotation and legs. I am 69 years old and can still push a fully loaded boat 30 or more kilometres a day at a stiff cruising speed without undue fatigue using this technique. As in all long distance endurance sports, running, XC skiing, cycling, etc., good technique is critical for efficient power, speed and endurance.
For this technique to work, the boat must also fit snuggly enough so you don't slop around. Your body must be comfortably upright, your hips and back should be held comfortably in place and your thigh and foot braces comfortably snug. Snug enough so you can use your legs to power your trunk, albeit not to the extreme shown in the linked video.
The type of paddle you use, Euro Blade, feathered or un-feathered, Greenland, Aluet will also affect the most efficient way to execute the stroke, Entry at the catch, high angle, low angle as well as the draw. But the basic principles of whole body power and torso rotation still apply universally.
That is the best I can manage to explain what I was taught, Learning proper technique from an expert paddler was the best thing to happen to me as a beginner paddler. It makes distance paddling fun and a pleasure, rather than a chore. You might even have a go at the odd rec race if one comes up. I always come home from a long paddling trip with a much tightened core and noticeably stronger back and abs. Lessons are not a bad investment for beginners if they can find a qualified instructor.
My two bits worth on this complex and perhaps somewhat controversial subject.
Cheers, Rick