Stir together rhubarb and sugar in a large saucepan (off heat); let stand until rhubarb releases some liquid, about 10 minutes.
Bring rhubarb mixture to a boil over medium-high heat, stirring occasionally. Reduce heat; simmer, stirring occasionally, until rhubarb has broken down but some whole pieces remain, about 5 minutes. Remove from heat.
Place ginger in a fine-mesh sieve set over a small bowl. Press down firmly with a spoon until juices are released (to yield about 1 teaspoon). Discard solids. Stir ginger juice into rhubarb mixture.
Let sauce cool completely before serving over ice cream, yogurt, or pound cake, or using to make parfaits.
