Cover the cooker and lock the lid in place. Turn the steam release handle to Sealing position. Select Manual/Pressure Cook setting. Select High pressure for 10 minutes. (It will take 8 to 10 minutes for the cooker to come up to pressure before cooking begins.) Once cooking is complete, let the pressure release naturally for 10 minutes. Carefully turn the steam release handle to Venting position and let the steam fully escape (the float valve will drop; this will take about 1 minute). Remove the lid from the cooker and carefully transfer the squash to a serving platter. Drizzle with the remaining 1 tablespoon maple syrup and, if desired, sprinkle with walnuts.