"Interestingly, according to David Merrick's biography, this rule happened because of what he pulled during HELLO, DOLLY. Whenever Carol Channing was out, he would wait until just before curtain and have the stage manager announce, "Mrs. Levi will be performed by [understudy]." He was banking on the fact that people wouldn't realize that Mrs. Levi was the starring role until too late, and he was right. After that, Equity required 2 of the 3 methods."
How can this be since it is on record that Channing NEVER missed a single performance of Dolly in its original run or its first revival. It wasn't until the 90's version she missed and even then it was only one or maybe a couple performances she missed due to a REALLY bad flu (I think). Maybe this was either his plan IF Channing missed, or this is what he did when the ladies who replaced Channing missed performances? But this definitely does sound like Merrick! LOL!
<-- Tevye, FIDDLER ON THE ROOF, March 2018