A year ago, we bought my brother's car using a box spread loan.

Actually, we didn't finance it with the box spread immediately.

The dealership was offering an attractive teaser rate for the first few months, so we took it. Once that promotional rate expired, we paid off the loan with a box spread instead.

Every dollar that would have been pulled out of his portfolio to pay cash for the car has remained invested and continued compounding.

Will this always work over short periods of time? Of course not.

Markets can decline. Borrowing to invest adds risk. There will absolutely be periods where you'd have been better off paying cash.

But investing is about probabilities, not certainties.

If your long-term expected after-tax investment return exceeds your after-tax borrowing cost, financing major purchases while you're still in your wealth accumulation years can be a rational strategy.

We've now used box spread loans for a variety of purposes, including:

• Refinancing higher-rate securities-backed loans or HELOCs
• Creating liquidity from concentrated stock positions
• Bridging quarterly tax payments
• Purchasing a home
• Funding a honeymoon