Good catch, that's a typo on my part. It should be additive.
On the math, well, I admit I wasn't clear on the point that it's the difference between the character's augmented maximum and the raised value. Sorry, this was written for my group and I hadn't gone over the language with a fine tooth comb to ensure the intent. I understand a little bit now how the writers feel after they see their rules interpreted by people. I also forgot to write it as "up to", allowing partial increases.
Anyhow, your example Spirit Burn wouldn't be necessary or helpful. If the Natural values are 4 and a Possession spirit possesses them, 8 is well within their augmented maximums.
So instead lets take an example of a human with [Bod 3 Str 4(6) Agi 6(7) Rea 5(
], possessed by a Force 4 spirit. Possessed the human will have [Bod 3(7) Str 4(9) Agi 6(9) Rea 5(9)], 9 being the augmented maximum cap. If Spirit Burn was used then all the human would become: [Bod 3(7) Str 4(10) Agi 6(11) Rea 5(12)]. For figuring the stress damage to host and spirit we tally the total points above the augmented max of 9, so 1 from Str, 2 from Agi, 3 from Rea. Our total damage per turn is 12P, resisted separately each combat turn.
The separate resists is so that the spirit could potentially kill it's host, and having a dead host means that it no longer is subject to attribute caps. I put that in so that it can be used strategically to kill opponents and enhance your spirit at the same time.
I'm going to work on the language of my little house rule and see if I can't clean it up a bit.