Spark's PSM is the stablecoin desk Sky's balance sheet runs away from mainnet: one contract that swaps USDC and USDS at exactly one to one and either of them into sUSDS at the savings rate's own conversion, in size, with no curve, no fee, and no inventory risk priced into the quote.

The rate it uses is not discovered by trading. sUSDS accrues the Sky Savings Rate every second, the rate is relayed from mainnet into an oracle the PSM reads, and every swap into or out of the savings token settles at that conversion to the wei. The only limit is the venue's own custody of whatever asset is being paid out, which is exactly what lets it quote parity to anyone with the asset on the other side.

Parity only helps the flow that actually takes it. The dollars sitting idle in a wallet that could have been earning the savings rate since the block they landed. The exit from sUSDS that went through a pool at a discount because the desk paying par was not the venue anyone checked. The rebalance that waited for a person when the condition was already true.

The PSM's custody and live conversion rate sit in the routing graph beside every other indexed venue, so a swap touching USDC, USDS or sUSDS on Base is compared against it automatically and fills there whenever par beats the market, and it runs when a condition is met rather than when someone is at the keyboard.

Idle stablecoins swept into the savings rate on a schedule. A position unwound into cash at the exact conversion the protocol enforces. A stablecoin leg of a larger route that takes par instead of slippage.

The dollars that compound on Base by rule are already routed this way, because the venue paying par right now is not something a person can keep in their head.