"You should make a jing, qi and shen sandwich."
In stillness jing and qi differentiate. Jing, in this case, is a feeling of underlying structure particularly as it relates to the limbs when they are relaxed--but also a feeling of continuous unified connection of the four limbs through the torso (via the four gates at the hips and shoulders).
Qi is very simply what causes our shape to change. It is expanding and condensing, fluid shifting and circulation.
If these two are mixed in your movement you will be like a sausage. Your training may produce superb techniques and agility by grinding together meat (jing) with some vegetables or fruit (qi) and then packing them into a piece of intestine. But when someone touches you they will feel meat, like a packed sausage. They will feel your jing. No matter how fast, clever, sensitive or relaxed you are, your opponent will feel your jing. When an opponent makes direct contact with your jing they can control you, and because of that, in order to fight you will have to alternate between going limp and then becoming strong.
When you make a sandwich you put the meat (jing) in between the lettuce, pickle, mustard, and mayonnaise (qi) and then put some bread (shen) on the outside. This way your opponent can never touch your jing (meat).
However, if you put your sandwich together in the wrong order you will make a mess. Not only will you touch the meat, but you will get mustard on your fingers, mayonnaise on the counter top, and you are likely to drop the pickle!
Shen, the bread in our little metaphor, is closely related to the yi (intent). Our yi (intent) must take a shape and a direction. The bigger and more sophisticated that yi is, the more likely we are to call it shen or spirit.
So the Taijiquan Classics should say, "The Yi (intent) leads the qi, which is followed by the jing. If the qi goes first, some gates will close and the movement will remain slow. If the yi leads the jing directly, the technique can be fast, sensitive, unified, dynamic and precise, but it can still be overwhelmed by an onslaught of superior power--because the jing will be exposed."
This idea finally clicked for me when I felt a student who was physically quiet enough for his jing and qi to differentiate, but when he went to move everything was in the wrong order--like a sandwich with the meat and lettuce on the outside and the bread on the inside. Most martial artists are somewhere between a hotdog dipped in mustard, cold cuts on a dry bun, or a sweet sage apple sausage with bread crumbs wrapped in lettuce.