"One of the soldiers with a spear opened His side, and immediately there came out blood and water" (Jn 19:34)
The followers of Jesus, including His Most Holy Mother, were Jews. Being thus, they wanted the body of Jesus taken down from His cross in order to prepare for and celebrate the Sabbath which was fast approaching. The bodies of those crucified could not be removed until they were dead. So, the soldiers stepped up to the scene in order to make death a reality - quickly. They made fast work of the criminals to Jesus' right and left, they then approach Our Lord.
According to St. Bonaventure, Mary at this time, begged the soldiers to not break her Son's legs. Trembling with fear she says: "My Son is already dead. Do not harm Him any more; I am His poor mother." While she was saying this, a soldier "rode up, brandishing a lance and plunged it through the side of Jesus. The cross shook when the lance pierced the body of the Savior and, as was later revealed to St. Bridget, the heart of Jesus was divided in two." Blood and water gushed forth - even these, perhaps the last drops of precious blood which remained to Him - He was pleased to shed for us. He had no more to give.
Jesus, His soul having fled His spotless body, did not experience the pain of this blow. Mary, his grieving Mother bore the pain of the lance. "Christ shared this wound with His mother. He received the hurt; his mother endured the pain."
Church Fathers maintain that this is the actual sword foretold by St. Simeon in his prophesy at the Presentation. This was a sword not of iron, but of grief. The soul of Jesus was one with the soul of Mary. Thus says St. Bernard: "The lance which pierced His side passed through the soul of the Blessed Virgin, a soul that could never leave her Son's heart." Mary indeed spoke of this to St. Bridget of Sweden: "When the spear was drawn out, the point was red with blood. It was then, when I saw the heart of my own dear son pierced, that I felt as though my own heart were also pierced." An angel later revealed to the same Saint: "Mary's sufferings were so great that it was only by a miracle on God's part that she did not die from them." Only now, she no longer has a Son to share her sorrow with. She is alone in the world for the first time in her holy life.
Let us go often to the foot of the cross with Our Lady and ask her to offer our small sufferings and sacrifices together with her sorrows to the Eternal Father. He is highly pleased with souls who honor His Mother's sorrows.
O sorrowful Virgin! O soul great in virtue but also in sorrow, for both were born of that mighty flame of love which you have for God, your heart can love no one but God and Him alone. Pray for us who have recourse to Thee. Be our salvation! Amen+
(Source: The Glories of Mary by St. Alphonsus de Ligouri)