The term is a nickname that fighter pilots have for the Valsalva Maneuver, a maneuver used to stay conscious in high G forces. A deep breath is taken and forcibly exhaled against a closed airway. You probably do it a dozen times a day without realizing it. You do it when you’re taking a dump and baring down, trying to lift something heavy or performing a difficult physical feat, try and not do it when weightlifting, it’s a bad form. If employed properly, it can seriously lessen the urge to flinch or jerk from a sudden pain. It works forcibly increasing the pressure in your chest cavity by inflating your lungs, holding your breath, and contracting the stomach muscles and diaphragm as if to forcibly exhale, without releasing the breath. This causes a dramatic rise in blood pressure to the outer extremities and head.
The best way to increase your pain tolerance is to get used to it. Practice by inflicting pain on yourself, while trying to keep a straight face. Do this whenever you’re alone, it freaks people out. Don’t cut yourself, while or prick yourself with needles, just stretch your legs. It serves two purposes, it makes you more flexible, and it inflicts a great deal of sharp pain for as long as you can bear it.
When the body is registering pain, and a second, more urgent injury is detected, it shuts off the pain receptors for the first injury to allow you to focus on more urgent injury, so if you have a broken leg, you splint it, but still need to walk, puncturing your skin with a sharp needle will lessen the pain of the leg. Many believe this is why acupuncture works as a means to manage chronic pain.
Your anger gives you focus, makes you stronger. It helps you endure great pain if you work yourself into a rage, by thinking about things that piss you off. Compose a montage in your head, set to heavy music, with scenes involving violent deaths of the people you despise. Hightened emotional states cause your brain to release adrenaline, making you capable of powering through physical distress.