Board Thread:Questions and Answers/@comment-38760351-20190309205647/@comment-29325824-20190309220427

Canon manga explanation? Fullbringers are humans infected with Hollow Reiatsu while in the womb causing them to develop their powers. So the insect sting analogy is indeed waaaay better than them being hybrids.

However, according to the Bleach: Can't Fear Your Own World novel, it's a bit more complicated. TL;DR, Fullbringers are concieved the way they are, because they are fused with parts of the Soul King's power that got mixed in the cycle of reincarantion. The Hollow's can somehow sense that power (probably on a subcionscious level) and their attacks (if the fetus and mother even survive, mostly thanks to Quincy or Shingami interference) merely awaken those dormant powers, not cause them.