بسم الله الرحمن الرحيم
The event of Shaq Sadar (splitting open the chest) and washing the holy heart happened four times throughout the life of the Holy Prophet Muhammad, may peace and blessings of Allah be upon him. Each event carried separate expediency. For the first time the chest of the Holy Prophet Muhammad, may peace and blessing of Allah be upon him, was split open when he, may peace and blessings of Allah be upon him, was staying with Hadhrat Halima, may Allah be pleased with her, and a dark clot was extracted out which in fact was the substance of sin from which the holy heart was cleaned. The second time it happened at the age of ten. It aimed to throw away the substance of play and amusement as it makes one negligent of the remembrance of Allah. The third time it happened when he was commissioned the mission of prophethood. It was performed at the time so that the holy heart is able to accept the secret of revelation and Divine knowledge. And the fourth time it happened during the Mi’raj (celestial journey). It was performed so that the heart could accept the journey of Alam-i-Malkut (the Dominion of Allah) and the observation of Allah’s Appearance and His Sings and the invocation of Allah and talking to Him.
(Summarised from: Siratul Mustafa by Hadhrat Maulana Idris Kandhalvi, vol. 1, p. 80, Chapter: Shaq Sadar Ke Asrar).
Allah knows Best!
Darul Uloom Deoband