Re: How do i know when i can feel KI?
FWIW, from a Chinese martial arts teacher of mine, the actual character for Ki is a wood burning stove with a pot of rice on top. The lid on the rice pot is jumping up as it cooks. I use the idea as structure (pot, stove body) and internal intention/imagery (fire) can lead to something more (hard inedible seed becomes nourishing and softer). So, steam, vapor, etc and something that could be defined by physics and psychology with extensive lectures but not helpfully. If your mind is on something else instead of your movement, then it's not coordinating with your movement - much as a debate on the nature of fire does not cook my lunch.
Kisshomaru Ueshiba's book on Aikido did have a dozen or so examples of how ki is used in Japanese language, and like has been said here it's not all Jedi Voodoo. It's just "Mind and Body working together can do more." Tohei's original work was on Mind and Body Coordinated, which is not so magical sounding.
Much the same as my first curling coach told me to keep my eyes on the rock when I threw it, and to try to see the throw before I made it. My cross country coach recommended we go home and see the course and rehearse in our mind, and my biathlon coach recommended all the same stuff - becoming a better shot wasn't only a matter of wasting millions of bullets. A boxer might not believe in Ki, but will talk about timing and targets, punching through the target, having your shoulder behind your hands, etc. Most Olympic sports do some version of mind and body exercises in training.
Unbendable arm is a use of imagery. In seeing yourself touching the wall behind your partner you are developing a mindset helpful in striking through a target, and it is a way to trick your body into relaxing muscles antagonistic to the motion - you have roughly four times as many muscles and ligaments as you do bones, and most are not consciously controlled by you or anyone naturally, but many can be, and usually indirectly.
If you are an athlete already, or a dancer, or have a focused mind and good structure, then you may never feel anything new, you would just have some new engrams to work on.
If you are wanting to have Kung Fu Panda Crouching Tiger Hidden Dragon Ki and want to know how to chase that, I got nothing.