Rule 18-2b states:
If a player’s ball in play moves after he has addressed it (other than as a result of stroke), the player is deemed to have moved the ball and incurs a penalty stroke.
The ball must be replaced, unless the movement of the ball occurs after the player has begun the stroke or the backward movement of the club for the stroke and the stroke is made.
This part of Rule 18-2b has not changed. What has changed is that an “Exception” has been added:
Exception: If it is known or virtually certain that the player did not cause the ball to move, Rule 18-2b does not apply.
This means that if you know your ball was moved by something else, such as a dog, a person or another ball, or you are virtually certain that wind or water caused your ball to move, you are not subject to penalty under Rule 18-2b.
If some agency such as a dog, person or ball moved your ball, you would need to go the appropriate section of Rule 18 to determine if there is a penalty to be applied. In all these cases, the ball must be replaced.
If wind or water moved the ball, there would be no penalty and the ball would not be replaced but would be played from where it came to rest. In general, it would need to be quite a windy day with fast greens to apply this exception.
What if the ball merely falls into a small indentation on the green after address? Decision 18-2b/11 states, “Gravity is not in itself an element that should be considered when applying the Exception to Rule 18-2b; therefore unless it is known or virtually certain that some agency other than gravity (e.g. outside agency or wind) caused the ball to move after address, the player is subject to a one stroke penalty under Rule 18-2b and must replace the ball.”
When Rule 18-2a Is Still Applicable
If you have not addressed your ball then Rule 18-2b would not apply, but you should be aware that Rule 18-2a (Ball at Rest Moved by Player, Partner, Caddie or Equipment) may apply. Rule 18-2a says that if you caused your ball in play to move you may be liable for a penalty. Rule 18-2a lists a number of circumstances when the player is not penalized, but in all other cases there is a penalty. Regardless, Rule 18 requires that your ball be replaced.
Frequently, players know they haven’t addressed their ball and think they can’t be penalized, but they may still have caused the ball to move. (For example, if you bounce your club off the ground 2 inches behind the ball and the ball drops off a small tuft of grass at that same instant.) Your action could easily have caused the ball to move and you should be penalized one stroke under Rule 18-2a.