Credit Enhancement, Subordination and Overcollateralization

Ask a bank to lend against a warehouse full of goods and it will lend less than the goods are worth — the gap is the bank's cushion if the goods turn out to be worth less than expected. Securitization uses the identical idea, formalized and made precise: build a cushion of extra collateral or extra subordinate money underneath a bond so that ordinary losses in the pool never touch it. Collectively, the techniques for building that cushion are called credit enhancement.

The main forms

• Subordination, covered in depth in The Securitization Waterfall and Payment Priority — junior tranches absorb losses before senior ones. This is "internal" credit enhancement: it comes from the structure itself, at no extra cost beyond accepting lower proceeds for the junior piece.
• Overcollateralization (OC) — the pool of collateral is larger than the bonds issued against it. Issue $95 million of bonds against a $100 million pool and there is $5 million of collateral with no matching liability, a cushion that absorbs losses before any tranche does.
• Excess spread — the pool's weighted average coupon is deliberately set higher than what is needed to pay all the bonds' coupons and fees. The leftover interest, the excess spread, is trapped in the deal each month and can be used to cover losses or to pay down bonds early (building more OC over time), rather than being paid out to equity.
• Reserve fund — a cash account funded upfront, or built up out of excess spread, held in trust and drawn down only if collections in a given month fall short.
• Third-party enhancement — a letter of credit or bond insurance wrap from a separate financial institution, guaranteeing the shortfall up to a limit. Less common since the 2008 crisis, when several of these guarantors were themselves downgraded.

Worked example: OC plus subordination together

A $500 million auto ABS deal is backed by a $525 million loan pool — 5 percent overcollateralization, or $25 million of collateral cushion before any bond loses money. On top of that, the $500 million of bonds is split into a $460 million senior tranche and a $40 million subordinate tranche.

Total credit enhancement protecting the senior tranche is the subordinate tranche plus the OC: $40 million + $25 million = $65 million, against a $525 million pool — that is 12.4 percent enhancement. If the pool's expected lifetime loss rate is 3 percent ($15.75 million) and its worst historically observed loss rate in a severe recession is 9 percent ($47.25 million), the senior tranche is protected even in the stress scenario, with $17.75 million of cushion to spare ($65 million − $47.25 million). That gap between the stress loss and the enhancement is what a rating agency is sizing when it decides a AAA rating is warranted rather than AA.

Worked example: excess spread building OC over time

Suppose the pool's loans carry a weighted average coupon of 9 percent, the bonds cost a blended 5 percent, and servicing and trustee fees run 1 percent. Excess spread is $9\% - 5\% - 1\% = 3\%$ of the outstanding balance, collected every month and trapped inside the deal rather than paid to equity (a common feature called a turbo or OC target structure). On a pool with an average outstanding balance of $400 million over a year, that is roughly $12 million of extra cash trapped over twelve months — enough, by itself, to double the OC cushion in the earlier example without a single dollar of new capital, simply by directing interest income to build the cushion instead of paying it out.

Where you meet it in practice

Credit enhancement is the number an analyst calculates first when looking at any structured deal: subordination plus OC plus reserve fund, as a percentage of the pool, compared against the pool's expected and stressed loss rates. It is also the lever sponsors pull to hit a target rating — want a smaller senior tranche's spread to be tighter, structure more enhancement beneath it; want to retain less capital as equity, structure less. Understanding this trade-off is the core skill of structuring, and reading it back out of a term sheet is the core skill of analyzing one.