I don't believe normal wrist motion converts the pendulum arc to a straight line.
pj
chgo
Not the wrist, the grip allows the cue to articulate by opening the back fingers.
OK, but that just allows the cue to move in its pendulum arc without interference from the hand. I believe the only way to convert the forearm's pendulum motion to a straight line is to move the elbow up and down - i.e., the "piston" stroke.
pj <- sometimes known as the "broken elbow" stroke

chgo
Pat:
While the opening/closing of the hand won't completely negate the arc effect of a pinned elbow, it *does* soften the arc.
As Lance mentions, the articulation of the back/rear fingers opening and closing as the cue is pulled back and pushed forward, respectively, prevents the hand itself from "participating in" (or "adding to") the arc movement.
Assuming a pinned wrist for the moment:
1. As the cue is pulled back, the hand position "tilts" with the arc movement, so the rear fingers become more prominent against/supporting the weight of the cue. This means there's more "lift" provided by the back fingers, which would ordinarily help raise the butt of the cue.
2. As the cue is then moved forward, the hand "tilts" in the opposite direction such that pressure is relieved from the rear fingers, and increasing emphasis is placed on the front fingers. So if the fingers don't move at all, they themselves will add to the "lift" and "lowering" (during pull-back and delivery, respectively) of the cue butt during the normal pendulum arc.
By articulating the grip -- opening it on pull-back, and closing it on delivery, the arc is "softened." This is enhanced by the type of grip that most snooker players use -- the "pincer clamp." That is, the index finger and thumb are the "pincer" part of the hand, while the remaining fingers are the cradle. By clamping the cue with the pincers, and opening/closing the cradle fingers, the arc is softened.
Some players will add to this "arc softening" by either A.) adding a bit of elbow drop or "bounce"; B.) manipulating the wrist to soften what remains of the arc.
Judd Trump is an example of a player that does both -- you can see in his cue delivery that, in soft strokes (only!), he'll add a bit of noticeable elbow drop or "bounce" -- what I like to refer to as a "scissor stroke." But he then softens/absorbs the rest of the arc by manipulating his wrist. In fact, that's where most of the "arc softening" occurs, because in power shots, he pretty much pins the elbow.
Notice the soft shot at 2:10 (linkcheat added, to take you right to the shot):
http://www.youtube.com/watch?v=KZA67elaLPI#t=2m10s
Notice the soft "bounce" in the elbow -- that "scissor stroke" I mentioned.
Now take a look at some power shots (linkcheats added):
http://youtube.com/watch?v=KZA67elaLPI#t=9m50s
http://youtube.com/watch?v=KZA67elaLPI#t=12m50s
http://youtube.com/watch?v=KZA67elaLPI#t=13m10s
Yes, when he's pulling the cue back, there is a hint of a scissor stroke (i.e. the elbow drops just a bit as he's extending his arm during the
pull-back), but when he snaps his biceps closed, that elbow doesn't move -- nor do any of the other major arm joints.
There's a lot going on to absorb the arc effect of the pendulum, but those built-in absorption techniques don't mean this isn't a pendulum stroke!
Compare Judd's cueing action to many other big-name players, and you'll see that Judd's elbow is remarkably fixed in comparison.
-Sean