So she drew 4 bamboo which should make her have a tenpai hand wait on either 1 or 7 bamboo and instead she cut the 4 bamboo and go back to iishanten. Then she somehow drew the 1 bamboo later on and make it exactly a suji trap kanchan wait on 6 bamboo which has only 1 tile left (could be in the dead wall btw) and it is exactly in youmu hand. She cant wait on 4 since that would put her in furiten and tsumo alone is not enough to overtake Youmu score.
Earlier, Youmu had 666789bam, called pon on the 6bam, and discarded 9bam, switching from a closed yaku-less hand to an open tanyao. With Remilia's skill, and apparent ability to see who is fated to win, she could definitely deduce that Youmu had the fourth 6bam all along.
Also, the 6bam wait gives her another way to win: chankan. If Youmu at some point ends in a yakuless tenpai, she might declare kan on the 6bam and hope to win by rinshan, at which point Remilia can ron the kan declaration for chankan, hatsu, nan, 6400 and win the game.
The 6bam also seems to be Youmu's fated tile in general. It's important to many of her critical hands. Is it some kind of you-mu = sou-6 pun, maybe?