Right, but you don't get the Yen, right?
It's like an option. When I short a call on Apple, I don't get $24k of cash to invest. I get a very small fraction of the notional (the premium).
When you short a futures contract, which is functionally just a series of TRS, you don't get the cash upfront, right? You just enter a contract for future settlement and commit to the margin associated.
So you can short $100 million JPY/AUD (obvious denominated outside of USD, just follow me here) and get $100 million worth of AUD.
But going short $100 million in JPY/AUD futures doesn't produce $100 million in cash to invest in AUD.
That's where my confusion is. It's just a large currency short. If it starts moving your direction, you get the variation margin, which can be invested, but it's still small relative to the notional and assumes the position immediately money in your favor.