It’s based on a (surprise!) weird interpretation on an out of context scripture from the book of Revelation.
Revelation 16:21 mentions “great hailstones” (weighing 45 pounds each) falling on people’s heads.
Official JW doctrine has it that these hailstones represent a “judgment message” that JWs will deliver in their door to door work just prior to Armageddon.
What’s that you say? The hailstones are mentioned as falling AFTER Armageddon, mentioned just 5 verses earlier?
Don’t bother me with details! The GB have thus decreed it! Who are YOU to question the spirit inspired ramblings of God’s only channel? Begone you sniveling worm!
Just as a side note, this “hailstone message” has a prominent part in this month’s JW Broadcast. It’s a little unusual, they seem to be implying that JWs will somehow start up this “hailstone message” very soon, even prior to the “great tribulation”. Which is kind of odd, since that would be a pretty significant change in doctrine.
Not sure if it’s just the usual “very soon now” fear mongering or if they are telegraphing a change in doctrine.