What's the rate of return that way? The 5% coming from the bank is pretty nice and is easily understood. I scrolled to the end of that BOXX page and even watched the video, and I still don't understand it.
The short answer is that if you could answer that question in advance to a high accuracy, you could make billions as bond trader. In theory, you get paid similarly to regularly going out into the bond market and buying US government bonds that expire in the next few months, but with a potential tax savings twist. (I am a random Internet dude, not a tax lawyer.)
The returns supposedly track the short end of the yield curve on US Treasuries. That would make sense, as theoretically, the net premium of a box spread is equal to the net present value of the payout (under the no arbitrage assumption). That net present value should be very close to the yield on a zero-risk asset over the same time period. They're using 1 to 3 month options, so in theory, they get yield close to short-term US Treasuries (the market prices a near-zero probability of the US defaulting on its bonds in the next few months).
I haven't looked into the tracking error between SPY box spreads and the short end of the US yield curve. https://en.wikipedia.org/wiki/Box_spread#cite_note-2 says the yield averages about 0.35% above holding equivalent maturity US Treasuries.
Though, it sounds like they're using box spreads composed of American options, so I wonder how they deal with early exercise risk. You only get bond-like performance from a box spread if you don't have early-exercise risk. The further out of the money they place their strikes in the box spread to avoid early exercise risk, the lower the liquidity they get, and higher trading costs.
The tax trick is that they also enter into a delta-neutral trade on a high-value single stock. (They don't use and index for this part because they want the difference between the winning and losing parts of this trade to be as large as possible, so they want volatility in the underlying asset.) At certain points, they realize the losses on the losing half of that trade (reducing tax liability), and perform a tax-free in-kind exchange of units (shares) in their ETF for the winning half of that trade. Of course, they don't know in advance which half will win and which will lose, but it doesn't matter. The brokerage buying their ETF in order to make the tax-free exchange bumps up the price of the ETF, very close to the value of the winning leg of the tax-saving trade.
Note the several caveats above (and probably some I missed) in comparing with US Treasuries yield.
Which is before taxes. Just like the 5% HYSA is before taxes. In the case of BOXX this is going to be 0-20% (depending on your income) whereas the HYSA tax will be 10-37% (depending on your income).
