What is the purpose of picking up ash?

Also known as climbing ash, it is a word to describe incest, especially incest between father-in-law and daughter-in-law

The story tells that Su Dongpo once passed by his daughter-in-law's room and saw her sleeping in a bed covered with a transparent gauze curtain, and her eyes could not help shining. After all, Wang Anshi is a poet, so he wrote a sentence on the dusty wall: "There is a pipa in the satin account, and I want to play it." After writing, hide aside and observe the daughter-in-law's movements. Seeing her father-in-law sneaking around outside, the daughter-in-law came out to see what his father-in-law had written on the wall. As soon as she saw his father-in-law leave such a message, she immediately understood what it meant, so she added, "I want to borrow my father-in-law to play a song, especially to leave Feng Shui at my house." Wang Anshi was secretly happy when he saw his daughter-in-law's words. I didn't expect my son to appear at this time, so he quickly wiped the handwriting off the wall with his sleeve. The son was surprised and asked his father what he was doing. Su Dongpo said he was picking up ashes.