The Priest Charges the Woman with An Oath of Cursing (Numbers 5:21-22).
‘Then the priest shall cause the woman to swear with the oath of cursing, and the priest shall say to the woman, “Yahweh make you a curse and an oath among your people, when Yahweh makes your thigh to fall away, and your body to swell, and this water which causes the curse shall go into your bowels, and make your body to swell, and your thigh to fall away.” And the woman shall say, “Amen, Amen.” '
Having previously received her oath of innocence he would now call on her to swear the oath of cursing (the parallelism confirms that there are two oaths). This probably began something like, ‘If I have been unfaithful to my husband, let this water of bitterness cause the curse to come upon me.'
The priest would then continue his words where he had broken off previously, “Yahweh make you a curse and an oath among your people, when Yahweh makes your thigh to fall away, and your body to swell, and this water which causes the curse shall go into your bowels, and make your body to swell, and your thigh to fall away.” The reference to her ‘thigh' almost certainly means her sexual organs. It was customary not to mention them by name, especially in public. The point is that if she was guilty she would by her words have called on herself the curse which would go into her bowels, and make her body swell, and her sexual organs be affected. This would then be evidence to all who heard of it that she was guilty and she would become a curse among them as unable to produce children. Furthermore her wickedness would in future be used as an oath because it was so heinous (just as someone might swear by Sodom when arguing innocence or when threatening reprisal).
The woman was then to respond fervently, ‘Amen. Amen. May it be so, may it be so.' In other words, ‘Let this thing surely be'. The willingness with which she said it would count for more than a thousand protestations of innocence.