John 19. 31-34
31 Now, since it was the Day of Preparation for the great Sabbath, and since the Jews did not want the bodies to remain on the crosses on the Sabbath, they asked Pilate that the legs might be broken and that they might be removed.
32 Then the soldiers came and smashed the legs of the first one and of the other one who had been crucified with Him.
33 But on coming to Jesus, when they saw that He was already dead, they did not smash His legs.
34 But one of the soldiers, with a spear, stabbed His side, and immediately came out blood and water.
We have already seen that it is not clear which Sabbath John is referring to. However, it is clear that the day on which Jesus was crucified was the day before a Sabbath of great importance.
It was the custom for the Romans to leave the crucified on the crosses until they died, which, as we have seen, could take several days.
However, this was an unacceptable practice for the Jews, since the Law of Moses showed compassion to the dying criminals. In Deuteronomy 21.22-23 we read:
 “If someone has committed a crime worthy of death and is executed and hung on a tree,
 the body must not remain hanging from the tree overnight. You must bury the body that same day, for anyone who is hung is cursed in the sight of God. In this way, you will prevent the defilement of the land the LORD your God is giving you as your special possession – (NLT).
From this we can see that there were two reasons why the Jews did not want the bodies to remain on the crosses: the Day of preparation, and because the Law required that no one should hang on a tree into the night.
It was in the light of this that the Jews wanted the legs of the crucified to be broken. This action would have hastened death.
The Romans would have used clubs to shatter the legs of the crucified. This was sometimes a separate punishment to criminals, and would have been extremely painful. With their legs shattered they would have no support for their upper body, including their lungs. Suffocation would soon follow.
The criminals either side of Jesus suffered this, since they were still alive. However, since Jesus was already dead they did not shatter His legs; instead they thrust a spear into the side of Jesus. This probably punctured His heart and lungs; the blood flowing from His heart and the water from His lungs.
It may seem ironic that the Jews considered that if Jesus was left to hang on His cross through to the Sabbath it would defile the Sabbath. Yet Jesus knew no defilement, any defilement was by the Jews, and by each one of us, since it was our sins that nailed Him to the cross.