What does it mean to let the man's father "pick the dust" when getting married?

There is a legend about the word "collecting ashes". Wang Anshi, the prime minister of the Song Dynasty, had an only son named Wang Kan, who suffered from andropathy and could not have sex. But after he found out, he was too embarrassed to tell anyone, so he found a friend who studied medicine to treat him secretly and was cheated of a lot of money. Because quacks mislead people, they don't know that they have been cured for life until they get married. He married a young lady from a big family named Pang Qixia. Not only that, but his father-in-law Wang Anshi also sympathized with him. The two expressed their feelings through words, and the communication between the ancient father-in-law and his daughter-in-law was suspicious. In order to avoid suspicion, Wang Anshi would bury a note in the incense in the ancestral hall every morning, and then take it out and bury a receipt when his daughter-in-law went to burn incense after he left. In this way, the two men maintained a deformed relationship. The word "picking up ashes" has become synonymous with incest between daughters-in-law.