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Back in 2019, the LPGA Tour found itself in the spotlight for a curious ruling involving two tour players and a practice called "backstopping." If you've never heard the term, tour pros commonly use it to describe purposely leaving a ball in place on the putting green instead of marking it. The thought is that it can potentially help another player who is hitting an approach shot onto the green. If the player off the green hits a shot that is too hot, the ball on the green might stop it from getting into real trouble.
Rule 15.3 covers this practice and, as you might guess, it's a penalty (two strokes). Getting back to the incident on the LPGA Tour, Ariya Jutanugarn and Amy Olson were paired in the same group at the Honda LPGA Thailand and reached the final hole of the second round with Jutanugarn's ball on the green and Olson's just off. Jutanugarn did not mark her ball and Olson's ensuing chip struck it. The incident got a lot of attention and scrutiny from LPGA officials because the two players fist-bumped on the green, suggesting there might have been some collusion.
The LPGA found in its investigation that the two players did not agree to leave the ball in place and they weren't penalized.
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Setting aside your opinion on whether they did or did not collude to backstop, there was a bunch of talk on the pro tours about the practice in the following months, with major championship winner Jimmy Walker saying he left his ball on the green from time to time to help other players.
What you need to know about Rule 15.3 is that in stroke play, regardless of whether you knew the rule or not, you can't agree to leave your ball to help another player—or vice versa. Ignorance is not going to get either of you out of a two-stroke penalty. Worse, if you both know it's illegal and backstop anyway, you both would be DQd under Rule 1.3b(1) for deliberately ignoring a rule.
Interestingly, however, in match play there is no penalty if you do it for your opponent. That's because "the outcome of any benefit that may come from the agreement affects only their match," a clarification (15.3a/2) to the rule states.
The smart move under Rule 15.3 is to mark and lift your ball if you think it might help another player who is about to chip. Keep in mind that in stroke play, you also can elect to hit the putt before the player chips, because there is no penalty for playing out of order.
Returning to the example of what happened in the LPGA tournament, procedurally if there was no intent to backstop and the ball on the green was struck, that ball must be replaced at its original spot. Meanwhile, the ball that hit the resting ball on the putting surface is played as it lies.