On back with a stability ball between your stacked knees over hips and hands pushing the ball into the knees. Extend opposite hand and leg and return to starting position. Is the back neutral or flat?Enter some more information that will help others answer your question
My personal training brain would chime in, “neutral is a range, not an exact point,” and would encourage the client to be as stable as possible (not changing the lumbar curvature irrespective of where they’ve positioned the pelvis.