Sitting with a slumped low back, or in postures where the angle between your torso and legs is less than 90 degrees forces the low back curve to straighten, reducing circulation and weakening support. This may lead to discomfort or numbness in the back or legs.
Self Test: Place your hand in the small of your lower back and alternate between slumping and sitting upright. Notice how your low-back curve goes away while slumping.