The Wikipedia article about Tirumalai Krishnamacharya, the "father of modern yoga", claims
During the 1920s, Krishnamacharya held many demonstrations to stimulate popular interest in yoga. These included suspending his pulse, stopping cars with his bare hands, performing difficult asanas, and lifting heavy objects with his teeth.
I had always assumed that the heart muscle was not subject to voluntary control, and thus stopping one's own heart would be an impossible feat, but there might be other ways to (seemingly) suspend the pulse as well, such as tightening blood vessels.
Is it physiologically possible for a person to "suspend their pulse"?