The term "burning" was no exaggeration.
The Realm of Bone Marrow Forging used one's Qi Blood Power as a flame to temper the tendons and fascia throughout the body.
The goal was to attain a state of gnarled, powerful muscles and Qi Blood as mighty as an elephant's.
During cultivation, one had to convert their condensed Qi Blood Power into kindling. This would ignite an intangible, true flame to forge the major tendons of the body, making them as hard as steel and as resilient as a bowstring.
Strong tendons unify one's strength.
Only then could one's power flow from the entire body, gathering all their strength to a single point to erupt in an instant, allowing them to lift the heavy as if it were light!
Once one broke through to the Bone Marrow Forging Realm, stopping a rampaging steed with one's bare hands would be child's play!
A casual kick could shatter the ground for yards around!
