There is no doubt about the context in which this appears. A great trumpet is going to be blown, undoubtedly the seventh trumpet. What will happen at the seventh trumpet? The context says that God is going to bring the children of Israel back into their land, showing God making a judgment that involves, not only the people of Israel, but also the land of Israel.
The word that is translated "thresh" is not the ordinary word for that activity. Ordinarily whenever threshing is done, the Bible shows the grain either laid on a firm surface and then beaten with a stick, or taken in hand and beaten against something solid, like a wall. The purpose for this is to break the wheat berries from the stock, and it generally takes a fair amount of force to do this.
The word "thresh" here does not indicate that kind of threshing but a method that is more careful and gentle. This word is applied when a person gently strikes an individual piece of fruit—like an apple, peach, or pear—from the branch, or when the tree is gently shaken so that the fruit falls out.
Here is God's judgment. At that time, the children of Israel will not be in a condition in which they will need to be beaten. Taking all of the scriptures on this together, we find that they will be returning to their land weeping, their wild spirit broken. It has been broken, of course, through the tribulation and the Day of the Lord. So as He is gathering, He is doing it one by one, leading them, as it were, by the hand.
At that time it is God's judgment that the children of Israel will need more than the usual amount of concern. He is indicating not just a separation from the nations, but that an act of purification is also taking place.