And He who sat on the CLOUD swung His SICKLE over the earth;
It is harvest time. Farmers will tell you they have to wait until the moisture content is below a certain level before they will combine a field. Here the grain is DRY (see previous verse) and READY.
The Lord Jesus begins the harvest with one swing of the sickle. The sickle probably is His word (the sharp two-edged sword - Revelation 1:16). It is a pure effortless swing of ultimate judgment. It is so easy for Him to perform this judgment act that He doesn't even have to stand up (like we probably would do to swing a sickle). He just sits there on the waters above the expanse (heaven - Genesis 1:6-8).
and the EARTH WAS REAPED.
This is what 2 Thessalonians 1:5-10 is all about. In the Old Testament, this is what is prophesied in Jeremiah 51:33, and relates this verse with Revelation 14:8 (as daughter of Babylon).
One swing of the SICKLE, and the EARTH WAS REAPED. How completed, how absolute, how FINAL this judgment is!