Narrated by Ibn Abbas:
'I (Muhammad) put on her my shirt that she may wear the clothes of heaven and I SLEPT with her in her coffin (grave) that I may lessen the pressure of the grave.She was the best of Allah's creatures to me after Abu Talib'... The prophet was referring to Fatima , the mother of Ali."The Arabic word used here for slept is "Id'tajat," and literally means "lay down" with her. It is often used to mean, "Lay down to have sex." Muhammad is understood as saying that because he slept with her she has become like a wife to him so she will be considered like a "mother of the believers."
This will supposedly prevent her from being tormented in the grave, since Muslims believe that as people wait for the Judgment Day they will be tormented in the grave.
"Reduce the pressure" here means that the torment won't be as much because she is now a "mother of the believers"after Muhammad slept with her and "consummated" the union. "After Muhammad slept with her and "consummated", Medina women routinely offered themselves to Mohammad for sex so that the holy fluid of the prophet released into them will qualify them for paradise.
Muslim Brotherhood has passed a law granting husbands permission to have sex with their wives up to six hours after the wife has died. It's exactly right. "Egyptian husbands will soon be legally allowed to have sex with their dead wives -- for up to six hours after their death. The controversial new law is part of a raft of measures being introduced by the Islamist-dominated parliament," under the control of the Muslim Brotherhood.