@GhostOnTheHalfShell the explanation i've settled on for myself about the various japanese carry trades is pretty simple:
1. ever since the 1989 crash Bank of Japan has loaned out money at insanely low rates (0% or even negative sometimes IIRC)
2. people like to borrow money from bank of japan and invest it in other stuff (like US treasuries) and pick up a small rate of return (e.g. if a japanese bank is charging 0.5% and treasuries pay 2% you can make 1.5% "for free")
3. because the rate of return is low people juice their returns by levering the fuck up and borrowing $50 for every $1 they're actually putting in
4. this amount of leverage means that even small movements in the value of the asset (e.g. US treasuries) blows people up (e.g. at 50x leverage a 2% move is death)
the specifics of each carry trade are hard to follow but BOJ has been regarded as a ticking time bomb for a long time now