Andrew Huberman· PhD
Actively exhaling, that is, actively relaxing the diaphragm and actively relaxing those intercostal muscles of the chest, those ones that are, I should say, between the ribs, is a skill that you can very quickly acquire and will allow you to use that relationship between the phrenic nerve, the diaphragm, and the size of the heart, the heart volume, and all that stuff to really take control of heart rate quickly.