Peel and cut sweet potatoes in half lengthwise, then cut each half into 1/4-inch slices. Place the slices into the pressure cooker's steamer basket. Sprinkle slices with salt.
Combine brown sugar, cinnamon, and nutmeg in a small bowl. Mix well.
Pour orange juice into the cooker (see Editor's Note, below). Place the trivet and the steamer basket with the sweet potatoes into the pot. Sprinkle the potatoes with brown sugar mixture. Place butter pieces on top.
Lock the lid into place and bring the cooker up to full pressure (15 pounds per square inch) over high heat. Once pressure is reached, reduce heat to low and cook for 7 minutes.
Quickly release the pressure using the method recommended by your cooker's manufacturer. Do not discard the cooking liquid.
Remove the steamer basket from the cooker and carefully place potatoes into a serving dish. Return the cooker to the stove and bring cooking liquid to a boil over high heat. Reduce the heat to medium and simmer, stirring frequently, until the cooking liquid is reduced and syrupy, 5 to 7 minutes. The mixture should coat the back of a spoon.
Follow the manufacturer's instructions and use the correct amount of liquid specified for your pressure cooker. For example, if the cooker requires a minimum of 2 cups of liquid, use 1 cup orange juice and 1 cup water. The extra cooking liquid will be reduced into a sauce on the stove top after the sweet potatoes are done.