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Since people have asked repeatedly, and I’ve obviously not explained it well enough, here is an explanation of the Heat Cycles in Fallen Grace.
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Okay, to give you the Hermione-version of the rundown…
First of all, the cycles are intrinsic to magic. I've built it as "normal" to the society, which is why it's just not discussed by others.
Mendax is latin for "false"
Geno is latin for "true"
Heat, either mendax or geno, occurs once every three months unless there is something to prevent it (typically extreme malnutrition would prevent a true heat cycle; however a mendax would still trigger). And one male in heat, without the suppressing potion, can trigger other males into minimum of false heat outside of their own cycle. That's why Harry was tripped by Severus. First heat typically occurs at the age of 17 about a week after the actual date of birth. And then it is a lifelong occurrence. Typically in a twelve month period, only one heat out of the four is a true drive to mate.
There are potions to help manage the symptoms and curtail the pheromone trails enough not to trigger other wizards or witches into heat. They are a regular part of life for the grown witch or wizard, and are just there for the population.
Harry, due to his power levels, more or less shorts out the potions. They just don't work for him. So, when Harry goes into heat, he only has his own will to help balance himself. Cold water will knock the need down for a little bit, and the rings help control physical arousal.
Before Severus, he would just hole up in Grimmauld and ride it out-- even though it left him sore, feverish, and irritable. With Severus, release does help. It soothes something in the drive to mate.
Harry has said once or twice, that to get a woman pregnant, he'd have to drug himself to do so. What he meant by that is he has zero sexual interest in a woman outside of a heat cycle. Hermione was a complete fluke because they were both in cycle at the same time.
Now as to "triggered" heat, if power levels were different, it would be entirely possible if seed and magic were to mix in a less powerful wizard to trigger a geno cycle in another.
That's what the handler meant. That's what Severus alluded to the first time with his question. That's also the reason that both Healers suggested condoms if Harry and Sev do have sex while either is in heat.
A cycle lasts two weeks. And they are regulated by magical power and body chemistry.
Harry does not express in the typical manner, which would be expected to find a woman he is attracted to. And to want to top. He also is perfectly happy with the way things are occurring with Severus, even while he is in the middle of a cycle. Let's just say that Severus will express closer to normal response when he triggers.
I hope this helps explain what is going on. Go ahead and ask further questions, ladies and lords.