The breech presentation mechanism involves the following key steps: 1. The left buttock engages in the left oblique diameter of the pelvis with a bitrochanteric diameter of 10cm. 2. The buttocks descend further through flexion and rotation, with the anterior buttock reaching the pelvic floor first. 3. The shoulders then engage in the same left oblique diameter, rotating to be born by lateral flexion similar to the buttocks. 4. Finally, the head engages with the sagittal suture in the transverse diameter, rotating to be born in a flexed position through external rotation of the baby's body.