The phrase "triangular bone" refers to a bone that has a shape similar to a triangle. It has three sides and three corners, like the shape of a slice of pizza. Full definition
This helps broaden and lengthen your lower back and sacrum (the downward - facing triangular bone at the back of your pelvis) to protect it in a backbend. (yogajournal.com)
Place your hands on your hips and put your thumbs on your sacrum, the broad triangular bone at the base of your spine. (fivepillarsyoga.com)
They are triangular bones, with two prominent spines making a T shape, held on the back by a matrix of muscle. (nakedpastor.com)