From time to time we discuss Ki, (or Chi, or Qi, depending on your discipline) in our training. While performing a technique, Sensei will have us work on drawing energy and projecting it into what we're doing. I have to admit, it seems to help. Whether it's actually Ki, or just the level of concentration that improves it, I'm not sure.
It is possible to root yourself and make it harder for your opponent to unbalance you, all while remaining in the same stance or position. Concentrating on becoming immovable makes it far harder to for your partner to shove, trip or throw.
I have had the opportunity to see a master who could make himself unmovable at a seminar. He invited students from any school to come and lift him off his feet. The master locked in a stance (a high stance, by the way) and had 4 burly young students (none of them his) try to lift him up. They could not. Now, I'm naturally suspicious that it was acting, but I knew one of the volunteers and he said he was really trying.
I also saw a Sifu sitting on a chair and four of his students, after concentrating, lifted him up, chair and all, using only their index and middle finger on one hand. They lifted him up to their shoulder height without any visible strain. I still can't say whether or not it was the Sifu making himself light or the students using Qi.
My cynicism says it's parlor tricks, my experience makes me think it's real. I want it to be possible and attainable.
I'm curious if anyone out there uses Ki in their training or have experienced it in their travels.