There was a confusing incident during Friday’s second day of the Presidents Cup matches in Incheon, South Korea. What happened actually is very simple but not so easy to understand because it involves a "Condition of Competition" that is used only at the highest levels of the game. The everyday golfer likely never has been subject to the one-ball condition.
The rule/condition in question can be found in Appendix I-C of the Rules of Golf. Understanding this ruling becomes more complicated because the condition involves an adjustment to the state of the match penalty for a breach. But before I cover that, let me explain the rule.