It's a bit long and complicated, so I'll break it up for you.
And the LORD spake unto Moses, saying ... If any man's wife go aside, and commit a trespass against him, And a man lie with her carnally, and it be hid from the eyes of her husband, and be kept close, and she be defiled, and there be no witness against her, neither she be taken with the manner; Numbers 5:11-13
The first thing to notice is the context. This procedure is only intended for married couples, specifically for any man that suspects that his wife has been messing around. No proof is necessary; suspicion alone is sufficient to God.
Then shall the man bring his wife unto the priest, and he shall bring her offering for her, the tenth part of an ephah of barley meal; he shall pour no oil upon it, nor put frankincense thereon ... And the priest shall take holy water in an earthen vessel; and of the dust that is in the floor of the tabernacle the priest shall take, and put it into the water ... And the priest shall set the woman before the LORD ... and the priest shall have in his hand the bitter water that causeth the curse.... (5:15-18)
OK, I am leaving some of the details out here, so if you're going to try this at home, make sure to follow God's instructions exactly. There's no guarantee any of this will work otherwise.
And the priest shall charge her by an oath, and say unto the woman, If no man have lain with thee, and if thou hast not gone aside to uncleanness with another instead of thy husband, be thou free from this bitter water that causeth the curse: But if thou hast gone aside to another instead of thy husband, and if thou be defiled, and some man have lain with thee beside thine husband: Then the priest shall charge the woman with an oath of cursing, and the priest shall say unto the woman, The LORD make thee a curse and an oath among thy people, when the LORD doth make thy thigh to rot, and thy belly to swell; And this water that causeth the curse shall go into thy bowels, to make thy belly to swell, and thy thigh to rot: And the woman shall say, Amen, amen. 5:19-22
This is the part that fooled me. I get the idea that if the woman has been unfaithful, then the magic bitter water will do something awful to her. But I wasn't sure just what. What does it mean to have your belly swell and your thigh rot? But then I saw the footnote in the NIV that said it meant this: "cause you to be barren and have a miscarrying womb."
So if the woman is guilty (had sex with someone besides her husband), then the bitter water will make her unable to have children in the future. And if she is pregnant at the time, it will abort the pregnancy.
And when he hath made her to drink the water, then it shall come to pass, that, if she be defiled, and have done trespass against her husband, that the water that causeth the curse shall enter into her, and become bitter, and her belly shall swell, and her thigh shall rot: and the woman shall be a curse among her people. And if the woman be not defiled, but be clean; then she shall be free, and shall conceive seed. This is the law of jealousies, when a wife goeth aside to another instead of her husband, and is defiled; Or when the spirit of jealousy cometh upon him, and he be jealous over his wife, and shall set the woman before the LORD, and the priest shall execute upon her all this law. Then shall the man be guiltless from iniquity, and this woman shall bear her iniquity.5:27-31
It's all part of God's wondrous Law of Jealousies. God's magical abortion procedure. A priest, some bitter water, and a wife that you think might have been unfaithful. Priceless. So if God has his own abortion procedure, abortion can't be wrong, right?