6.1 Cue ball fouls only: When a referee is not presiding over the match, One Pocket may be played whereby disturbing a single object ball is not a foul, as long as there is no effect on the shot. For clarification, it is always a foul to disturb the cue ball, and it is also a foul if a disturbed ball has any effect on the shot; meaning if the disturbed ball has contact with any ball in motion, or if any ball passes through where the disturbed ball originated. It is always a foul when two or more balls are disturbed. When multiple (3 or more) balls are disturbed it is considered a serious foul. ref: 6.6.2 Serious fouls
6.1.1 Restoring a position: There is no restoration option when any disturbed ball has had an effect upon the shot; in that case the balls must be played from where they lie. Once it is acknowledged that a ball or more was disturbed with no effect on the shot, then the opponent must be given the option to restore the position or leave the balls as they lie before play is resumed. If the opponent elects restoration, it shall be as near as possible to the original positioning according to both players, with the opponent responsible for final approval. It is a foul upon the shooter to restore a ball without permission from the opponent. However, if the shooter was notified, yet resumes play without the opponent's consent to the final disposition of a disturbed ball, then it is a breach of the restoration option, and a serious foul on the shooter. ref: 6.6.2 Serious fouls
6.6.2 Serious fouls: If the ruling is that a serious foul has occurred, in addition to the standard foul the official may further penalize a player at their discretion. If there is no official available, players will need to come to agreement themselves as to the level of penalty to assess the shooter. If it is possible to restore the balls, then the foul may be penalized (a) as a standard foul, with opponent's option of restoration.
The following penalty levels progress from (a) through (c) depending on the seriousness of the offense, and whether the shooter has been issued a prior warning. A prior warning warrants an escalation of penalty. In this context a “prior warning" may mean a prior offense, a pre-tournament announcement or a player agreement prior to a match.
(a) Assess a standard foul penalty, and a warning to the shooter.
(b) Assess a standard foul penalty, and opponent receives the option of ball in hand.
(c) Loss of game.
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Here are Steve's rulings when a ball or multiple balls, or two balls are disturbed. Some disturbed balls have an effect upon the shot, and others do not. The final sentence of 6.1 Cue ball fouls only; When multiple (3 or more) balls are disturbed it is considered a serious foul. ref: 6.6.2 Serious fouls.
This does not distinguish between whether the disturbed balls has an effect upon the shot or not, it just states when 3 or more balls are disturbed, so then you travel to 6.6.2 Serious foul, to determine the appropriate action. A referee has full discretion, a member playing a member has to decide amongst themselves as to penalty given to their fellow opponent.
But, it is suggestive but not definitive that the players 'may' enact a standard foul, with the opponent's option of restoration, if they feel the balls are restorable.
But, as we look at 6.1.1 Restoring a position: to find the answer, we see there is no restoration option when there is an effect upon the shot.
So when 3 or more balls are disturbed when there is an effect upon the shot, and when there is no referee presiding, you and your opponent then go to (a), (b), or (c) to administer the appropriate penalty.
(a) Assess a standard foul penalty, and a warning to the shooter.
(b) Assess a standard foul penalty, and opponent receives the option of ball in hand.
(c) Loss of game.
I hope I got this right! But this is on my part a simple exercise for future rule questions that will undoubtedly arise.
But, to answer the question; when a disturbed ball effects a ball set in motion by the shot, or when a ball set in motion travels through an area once occupied by the disturbed ball, then it is a foul.
Whitey
A good rule of thumb; when you disturb a ball and 'know it', stop and acknowledge it, otherwise the disturbed ball may result in an effect upon the shot, and that is a foul. If it is in the act of shooting, well there is nothing you can do about it, and live with the results.
But, under Steve's rule a ball disturbed that contacts another stationary ball is a foul, but can be restored. But when it gets to 3 balls or more disturbed then ref. 6.6.2.
But not to worry about this to much, for I have never seen 3 balls disturbed, pretty rare.
Note; it is commonly thought when shooting over a ball and the ball moves towards the cb it is a foul, and away from the cb it is not a foul. Not so in either case, unless the ob contacts the cb, or the cb once again goes through the area once occupied by the ob.
That forwards stroke contact or backwards stroke contact does not apply.