When you sit down to breathe, notice your pelvis. Tucked hard under or tipped hard forward both squeeze the breath; somewhere in the middle lets the diaphragm move.

What the research says

An integrative review of heart rate variability explains how closely breathing pattern and heart rhythm are tied, and why that variability is read as a sign of the body's capacity to adapt. The link between a neutral pelvis and easier breathing is anatomical common sense rather than something this review tested. So the breathing part is well grounded and the posture part is reasonable inference.

Strength of evidence: long taught, never tested. Widely used and long taught. No trial has established that it works.