In Pilates breathing, you want a solid, stable frame for the torso so the breath can nourish the spine and core without the rib cage buckling or flaring. The idea behind lateral rib expansion, in this context, is that air should fill the sides of the rib cage without the chest lifting or the ribs moving in uncontrolled ways. Keeping the ribs relatively rigid provides that stable platform, allowing the diaphragm and the deeper core muscles to work effectively to support spinal length and pelvic stability. When the rib cage stays steady, you can recruit the transverse abdominis and obliques more efficiently, which helps maintain neutral alignment while you breathe. If the ribs widened or lifted excessively, it would introduce unwanted movement that can derail the core engagement and spine stability. So the best cue is to preserve a steady rib cage while the breath expands into the sides and abdomen, rather than relying on chest elevation or wholesale rib expansion.