‘So he brought down the people to the water, and Yahweh said to Gideon, Everyone who laps of the water with his tongue, as a dog laps, him you will set by himself. Likewise everyone who bows down on his knee to drink.” '
The distinction was to be between those who took water in their hands and lapped it like a dog, and those who knelt down and put their faces in the water, not noticing the leeches.
It may be that this was simply a way of distinguishing a small group from the remainder, but there may well have been more to it than that, for Yahweh had described it as a ‘test'. Those who put their faces in the water showed a certain lack of self-restraint and of alertness. Furthermore they demonstrated that they had not noticed the leeches (see on Judges 7:1). Thus they had less control and were less aware of things. For what he was about to do Gideon needed men of iron control and men who had their wits about them and were alert.