‘And Yahweh said to Gideon, “The people are still too many. Bring them down to the water, and I will try (‘separate by refining') them for you there. And it shall be, that of whom I say to you, this shall go with you, the same shall go with you, and of whoever I say to you, this shall not go with you, the same will not go.” '
Gideon was told to take them all down to the water's edge, where Yahweh would separate those who were to go in the first phase from those who were not to go. Notice that Yahweh's purpose was to ‘test' them. This was thus a refining process in order to obtain the most useful.