Right, so I have listened to the interview and, while there are some things that Bass said that I certainly agree with, there's not much detail and it's, obviously, him presenting his side of the story. So, in order to find more detail, I have gone and re-read one of his famous investor letters from a while back (Q3 2009) where he talks about Japan a bit more.
The analysis that he offers to support his "short Japan" thesis is superficial, IMHO. It doesn't address several important issues.
1) Japan's running a current account surplus, even with yen pretty much the strongest it's been.
2) Japan's headline govt debt-to-GDP number is overstated, due to the intra-govt cross-holdings of JGBs and FILP bonds.
3) The rise in the corporate savings rate that has offset the often mentioned fall in the household savings rate.
4) There seems to be specific confusion about what the yen exchange rate actually does to the Japanese economy. On the one hand, his expectation is for weaker yen and yet, mysteriously, he cites that a strong yen is going to be a problem for exporters and thus will negatively impact GDP. I am not really sure I understand how you can have yen both weak, so that you profit on it, and strong, so that it actually causes a problem.
5) Finally, in the investor letter, Bass explicitly gives his timeline for when he expects the Japan trade will start performing. He says and I quote "We believe global OECD rates will begin their ascent over the next 18-24 months and that the best convexity for rates is in Japan". The letter is dated Oct 2, 2009 and 10y JGB yield on that day was arnd 1.30%. 10y JGB yield today, more than 24 months later, is 0.94%. During the 24 month period the high was 1.5%, the low 0.85%. I have already mentioned the negative carry. So, clearly, Kyle Bass doesn't quite have the timing of this JGB trade right, which is exactly the problem with so many of his famous predecessors.
At any rate, I have no way of knowing how exactly the trade is structured. Given that he mentions convexity of rates, I presume it's through options. It's entirely possible that Bass has found a fantastically cheap way to put these trades on, but there doesn't seem to be any evidence of that.