The one we've been waiting for, Hollow Flame is the Initiate Rank variation of the Will to Survive Mode, Atavistic Flame's Sage Mode equivalent, functioning identically to the higher rank variations, but with lesser increases to the user's attributes. As noted, Atavistic Flame is a representation of Humanity's preservation instinct and as such is at it's strongest when the user is in a dire situation, with a greater desire to live.
For this Mode, the user activates it when they are missing health points from their total, which is then multiplied by 4.5x. So for example, if the user has 100 health missing from their total they will have an effective pool of 450, while if they are missing 150, it's increased to 675. The more health the user has, and the more they are missing, the greater the result will be when they enter Will to Survive, especially in the higher ranked variations. Once the pool is expended the Mode ends, and it is maintained at a cost of 10 per turn. The main saving grace about this mechanic is the fact it's exempt from fatigue rules, and this extends to the user themselves while in the Mode.
With the pool explained, next would be how it's used. If a technique were to inflict recoil on the user, that still applies, reducing their health by the given amount, and recovered following that same mechanic. Outside of this, the "effective Chakra" from a technique is deducted from the user's total. For example, if the user were to have the pool of 450, and performed an A-Rank application of Human Principle, then the "effective Chakra cost" of an A-Rank would be deducted from Will to Survive's pool, reducing it by 30, and leaving the user with 420, from which the -10 per turn cost also occurs, leaving the user with 410 left for the mode at the end of that turn. That's how it reads to me anyway, though I do have to ask if the additional Chakra these techniques get in clashes with Anutu and it's components are supposed to be included within these calculations? I think they don't, but better to ask and be sure.
In return for this unique mechanic that requires the user to essentially flirt with death, the user gains several benefits. Firstly they gain a -20 damage mitigation for all forms of damage, physical or spiritual. They also gain the ability to sense all sources of life presumably within the same Landmark, and at long range otherwise, with a potency of x2. On top of this, the user's Atavistic Flame techniques gain a +20 to their damage, as well as automatically recovering missing health at a fixed rate, regardless of type, physical or spiritual. The numerical value of these boosts are standard for most techniques, but naturally increase in potency with the Master and Grandmaster equivalents of Will to Survive, and the actual effects themselves are very nice, and are what you expect from a proper Mode.
Overall a very nice Mode, and one I hope I don't have to wait too long to use. No questions aside from the Anutu additional Chakra question.