Hybrid publishing happens when an author has traditionally published and self-published works in the market.
This is happening more often as traditionally published authors decide to also self-publish. It also happens when successful self-published authors are offered traditional publishing contracts.
Katheryn LeVeque has self-published 70+ medieval romances. She just accepted a contract with SourceBooks to traditionally publish a series of Scottish romances.
What to "go hybrid"? First, show strong sales with your self-published works. Like millions of copies. And free doesn't count.
A SourceBook editor explained that they invest a lot into the publicity of an author. Lower priced titles by the same author can cannibalize sales of the title they were trying to promote. And a publishing house gets nothing if a reader buys the self-published book.
Hybrid deals often involve a new genre and a price fixing deal. But it can be a great opportunity for authors.