‘ And you shall command the priests that bear the Ark of the covenant, saying, “When you come to the brink of the waters of Jordan, you shall stand still in Jordan.” '
The priests were to be commanded to go forward with the Ark of the Covenant and when they came to the brink of the waters they were to enter and stand still with their feet in the waters (Joshua 3:13). And while they stood there YHWH would cause the waters to cease flowing.