Shielding is a rather diffcult to put in simple terms but I will try and confuse .....
The shield is nothing more than an barrier to keep something out, something in, or take something away.
The barrier can be solid, like a metal wall, or it can be open, like a fence and still act as barrier. Thus electrically
it can be 100% or less. It can be open, and be effective, but the actual opening size is frequency related.
The best shield is 100% as it totally shields what is inside from what is outside and vise versa. Additionally it can can conduct.
So what happens to the shield at the ends is important.
If one end is connected to ground and the other not connected, then it acts as shield along it's path but additionally it conducts
and provides a path for signals to the ground. It won't transmit all signals since depending on it's length is acts as a short to
transmission.
Shields are normally taken to a single point and that point goes to ground. Ground is not ground. Ground is just a reference.
In the case of shielding, ground is taken to a single / common point and usually that single point continues to a specific point not
associated with other points of an electrical device. Simple said, the shielding is run to a point which provides almost no resistance to
the signal and separate from any other grounds. If grounds are connected then signals can can travel along to what that other ground
are connected to.
BTW, you twist wire pairs as the signals running on the outside of the conductors have difficulty mixing or traveling since an inductance is
seen by the signal.
Now it gets more interesting as one must consider how signals are received, mixed, transmitted, etc, etc.
A conductor can receive a signal via conductance or inductance. Any conductor which cuts an electrical field will acquire a signal.
The shield will keep that signal out from combining with or riding on the inside wires.
I have used signal rather loosely. The unwanted signal is usually some kind of field created by say a motor, vfd, etc. and that field is
made up of numerous signals of varying frequencies. Noise as it is called, because we can't understand the meaning as compared
to speech or represent it as a simple wave.
How intense a field is rather difficult to envision as you can't see it,but, for description purposes just envision the field as a balloon
filed with signals and the source of the signals is at the center of the balloon. The noise weakens in intensity as the distance increases.
So use the above to understand how shielding is used when running wires to a motor or whatever.
FWIW,
RICH