Most ki-orientated Aikido schools demonstrate Ki energy by using an exercise called the Unbendable Arm.
This is where an Aikido practitioner stands face-to-face with a partner (or member of the public), places his bowed arm so the wrist rests on the person’s collar bone, and then asks that person to use both hands and try to bend his arm at the elbow.
The first time the exercise is done, the demonstrator will resist with muscular strength. He has two arms working against his one, so he will almost certainly lose; the arm will be bent quite easily.
For the second part of the exercise, the demonstrator relaxes and imagines his center of gravity to be at his One Point. He further imagines that his body is charged with Ki Energy which flows from his One Point through his body, right along his arm and out through the tips of his fingers. He imagines his fingers are like a fireman’s hose, and that his Ki is extending, shooting like jets of water out to Infinity.
With this mind-set, and extending Ki Energy, the demonstrator’s arm becomes unbendable.
The person trying to bend it should not be able to do so, even with both hands and all his strength.
Even with a beginner trying this Ki exercise for the first time, the arm may be bendable, but a distinct difference between Strength and Ki Energy should be felt.
And the skill gets better with practice (and confidence).
Many people believe that George Lucas, the author of Star Wars, borrowed the idea of Ki Energy when he coined the phrase, ‘May the Force be with you’.