Heat Gauge (ヒートゲージ Hīto Gēji) is a gauge located at the bottom of the screen, which is the series' primary "Super Move resource" bar. Characters can use this gauge to execute Distortion Drives, Astral Heats, Crush Triggers, Counter Assaults, and Rapid Cancels.
It can be charged up to 100% via being split in half with each side being 50%, though as of Chronophantasma it is now divided by four per 25% instead, and is filled when a character lands an attack or suffers a hit as per many gauges that follow this scheme in a fighting game. However, the amount of Heat gained per hit scales downwards as a combo goes on, and some attacks (especially Distortion Drives) whenever they are performed will force the gauge to enter a cooldown period (specific to the move used in question) that prevents Heat from being built for a short time; something unique to the BlazBlue series as a form of balance. The Heat Gauge resets to empty at the start of each round. When a character's health bar is under 35%, their Heat Gauge will slowly gain Heat automatically at the rate of 1 unit per frame.
However, in Hakumen's case (where it is considered to be a "magatama gauge" in some circles), he normally gains 3 units per frame (with one magatama being akin to 10% heat), but when he has less than 35% health, he gains 4 units per frame. The Heat Gauge will have a flashing icon when this occurs. Both Jin and Hakumen are the only two who use the Heat Gauge for their special moves as opposed to the other aforementioned mechanics; in Jin's case, he was the first character to have his gauge split into 4 before Chronophantasma due to his usage of EX Special Moves through his Drive mechanic. While in Hakumen's case, all of his special moves use a specific amount of magatama.