My personal expereince of Qi is the increased blood flow to the extremities, thus the muscles of the forearms and the fingers expand like they have been pumped full of blood like a balloon. Also I in zhan zhuang I can feel the resistance in the ball (magnetic field?) between my fingers and my arms.
My theory is that increased solo training increases the blood flow to the muscles / fascia and strengthens the nerve signals and connectivity, supporting and facilitating the connections in the body. Thus when my Qi is strong it is easier to establish a connection to the ground. When I am sick or when I have been indulging in too much sexual activity, my body is sluggish and my qi is affected - it is difficult to increase the blood flow through intention and I am easiy pushed over / collapse.
Also the muscles / fascia in their expanded rubbery state act as good protection for the internal organs, like in iron shirt qigong. It is like wearing a a rubber suit and some of the impact is asorbed or bounces off.
My personal attempt at a very rudimentary description of what I feel when I train qi. I may be wrong and off the track completely.
I do believe there is a whole area of using the Qi to lead your opponent and to take his centre or attention which I am not proficient in yet and am not able to comment.