In addition to the above comments, I'd say that there are two parts to the equation:
1. The source of power (that drives all movement, including technique)
2. The martial application (i.e. martial conditioning)
The former is the engine. In an "external" system and conventional athletics, power is generally derived from a combination of twisting at the hips, upper back and shoulders, forward momentum (stepping), pivoting on the foot (using a jerking twist of the body to propel it), dropping the body (gravity), and using the muscles of the upper back and shoulders to whip or propel the arms. In an "internal" system, power is derived from creating dynamic tension within through opposing forces, expansion and condensation of muscle and connective tissues -- both surface layer and deep -- re-direction of force through aligned joints, and a relaxed structure that permits high levels of potential energy from which to draw.
Martial conditioning for the two approaches does have some overlap, particularly in the most pragmatic factors. For example, there are certain ways to make a fist or hold the hand, foot and other striking surfaces, in a manner that will not result in injury for the user. There are certain ways to strike specific targets, such as using your soft parts to strike a hard target, and hard parts to strike a soft target. Of course there are always nit-picky exceptions, but in general these are universal concepts for martial application.
Those things in themselves are not techniques; they are the martial conditioning. In addition, have a general and working knowledge of basic punches, kicks, joint locks, throws... just basic skills in how to pull off a koshinage, a rear naked choke, etc. Know where the nerve points and joints are and how to attack, control and (for oneself) protect them. You don't need to memorize a vast curriculum of technique combinations, just do the conditioning for those very basic individual applications.
As has already been pointed out, when your body is conditioned for IP/IS and aiki, that conditioning itself will respond to changes in the combat environment (i.e. what your opponent does, what the situation provides) naturally, and whatever martial conditioning you have in your kit will kick in and make for spontaneous creation of "technique."
That's what Ueshiba did. It's what his aiki teacher, Takeda, did. It's why it looked like they never did the same "technique" twice... and why their students frantically tried to write down and record everything the two men did, thinking that they were seeing a vast, memorized encyclopedia of techniques that should be codified into a curriculum for students to painstakingly memorize. If those students could "simply" have learned the body method, they would have understood where those "waza" were really coming from.
A few baseline martial application skills, infinite combinations...all driven by one distinct method. From one thing, ten-thousand things.