Radak #1, Targum Yonasan: It is his speech, like it says "Sefaso

Rashi: Anyone who passes in it can go only until the water reaches his neck. He cannot stand against the flow.

Radak: When one is up to his neck in a [flooding] river, he has no way to escape. Either he will drown, or he is close to drowning. So Machaneh Ashur was almost totally wiped out. Only a little remained, like the head [is miniscule compared] to the body.

Rashi: It is to sift in a vain sifter - it does not help at all.

Radak: Lahanafah is Makor (like an infinitive), like Lehanif. His Ru'ach will come to winnow the Goyim in Machaneh Ashur. Winnowing chaff from grain is useful. This winnowing is futile - nothing needed will remain. All will be lost.

Rashi: It is the bit (of a bridle). Malbim - a bit makes the animal go on a straight path to where the rider wants to go. Hashem's Ru'ach will make them stray, and lead them to destruction.