Key Spreads 1 in IRS refers to an optional 1D-array of predefined constant rates s that must be added to the product of the appropriate gearing from Gearings 1 and the applicable fixing in Index 1 in order to produce the final rate used in calculating the floating cash flow amount.
If omitted, a flat value of 0 is assumed.
If n values are provided, then the first n cash flows are paired with the provided n values in successive order.
If any cash flows are left over, they will use the last value.
Look at IRS for the exact mathematical formula.