The allusions of ash removal

It is said that Wang Anshi, a great writer, has a daughter-in-law at home, who has a beautiful appearance. On this day, Wang Anshi passed by his daughter-in-law's door. He saw that his daughter-in-law was taking a nap, with a light veil and a half-dew, and a beautiful body. He couldn't help but feel excited. He was eager to be intimate with his daughter-in-law, but he was afraid that she would refuse. He murmured a little, coughed, and wrote a poem on the wall:

There is a pipa in the red Luo account, and I dare not take it when I want to play it.

The daughter-in-law heard the sound and went out of the room to check. When she saw the inscription of her father-in-law on the wall, her face turned red. She pondered a little and continued with a pen:

I would like to borrow my father-in-law to play a song, and the fat water will not be left to others.

Seeing this, Wang Anshi was overjoyed and was about to take action. He went to Wushan with his daughter-in-law. Suddenly, he heard the sound of footsteps. His son passed by and asked, "What is your father doing?"

Wang Anshi was shocked and thought, "The poem on the wall can't be seen by his son." Otherwise, the evening festival will not be guaranteed. He had an idea and said with a smile, "I'm picking up the ashes." Between the words, he stretched out his palm and cut out the inscription on the wall as if nothing had happened, leaving a word.

This is the allusion to pick up the ashes. What does it mean to pick up ashes? The father-in-law steals his daughter-in-law.