The Interpolate tool interpolates strokes between the previous and next keyframe by adding a single keyframe. When you are on a frame between two keyframes and click and drag a new breakdown keyframe will be added. This way you define the final interpolation for the new stroke.
Set the Playhead on the Timeline between the two keyframes you want to interpolate. Click and drag from left to right to set the desired interpolation percentage and release to confirm, a new breakdown keyframe will be added.