There is only one POINT in a pendulum arc where it is not either moving down or moving up.
Since it is a single point the term moving can not even be legitimately applied to it.
If the hand on the cue is moving up then the the tip is moving down. If the hand on the cue is moving down then the tip is moving up.
In order to limit such there would need to be a changing connection to the cue &/or wrist action that is not present in a true pendulum.
If the cue is sitting in the 'cradle' of the four fingers the resting point of the cue would have to constantly change as the hand is swung up on the back 'swing' & down on the forward 'swing' & again as the hand is swung up
during contact.
The approximate 3 to 4 inches of available change on the 'cradle' is insufficient in length to off set the rise & fall of the hand. That length of change can merely reduce the rocking effect of the cue stick & the subsequent up & down movement of the tip on the END of the cue stick
Hence the tip arcs down as the hand 'swings' up & arcs up as the hand 'swings' down & arcs down as the hand 'swings' up again.
There is a single POINT where there is a change of direction from arcing up to arcing down where the tip is doing neither. As said earlier, if it is a single point, then the term moving is inapplicable to it.
To get the tip on the END of the stick to make contact with the ball at that single point where it is neither moving up nor down would take a PERFECT set up. Human Beings are rarely, if ever, perfect.
So what then MIGHT get the cue stick & the tip moving in a straight line for even a very very short distance?
Well, the only thing that is available is the wrist. If the wrist goes into the radial position on the back swing in order to limit the effect of the pinky finger lifting the stick then the index finger rises & lifts the stick.
That would then need to be reversed as the hand 'swings' down & then the hand would need to go into the ulnar position to lessen the effect of the index finger lifting the cue but that puts the pinky finger into a lifting position.
That wrist action is what CJ Wiley referred to as his hammer wrist action.
As I have said what is now many times, the connection to the cue must fit the stroke BUT that can only do so much in trying to get the cue to move in a straight line any more than for just that single point.
What is available is a complex coordination of the cue stick changing resting points on the hand/cradle while the wrist is going through the changes of neutral to radial, back to neutral, & then on to the ulnar position.
That is not the wrist action that most envision for the stroke as most would see it as just the opposite.
The stick is straight & we use the end of it with a small leather tip to strike the ball.
To 'swing' the stick similar to how a pendulum swings is not conducive & best for the function of the task desired.
The stick is straight, why not move it in as near to a straight line as humanly possible?
Keeping the elbow still while the arm beneath it is moving is also not a natural thing to do. So why go against nature & what is bio-mechanically natural?
If the cue is rocking it would take PERFECT timing of when to release it to get it to move in a straight line as it rests on the bridge hand for the release stroke or stroke slip or slip stroke whatever the the released cue stroke is called.
BUT... if it is already moving in a straight line then one can release it at any point in time & it would move in a straight line.
Pendulum ARC vs a Straight Line like a piston in a cylinder.
All of the above is just food for thought.
Best Wishes to ALL.