“Then Absalom said to Ahithophel. ‘Give us counsel. What shall we do?’ Ahithophel said to Absalom. ‘Go in to your father’s concubines, the ones he has left to keep the house. All Israel will hear that you have made yourself odious to your father. The hands of all who are with you will be strengthened. So they pitched a tent for Absalom upon the roof. Absalom went in to his father’s concubines in the sight of all Israel. Now in those days the counsel that Ahithophel gave was as if one consulted the oracle of God. Thus all the counsel of Ahithophel was esteemed, both by David and by Absalom.”
Absalom asked Ahithophel for counsel on what to do. He was considered the best for counsel, like a Greek oracle. His advice to Absalom was to have sex with the 10 concubines that David had left behind to take care of the palace. He felt that public sex with his father’s concubines would show who was in charge. This would embolden his followers and make them stronger. So then they put a tent on top of the roof so that all would see the new king Absalom having sex with David’s 10 concubines. It was not clear whether this was all at once or sequentially. Notice that the names of the concubines are never mentioned.