Self-insert describes reading from inside the protagonist, so that her feelings, choices and glances are experienced as your own. In romance that's not a side effect but the point: you read to be the one who is meant.
Fan communities have used the form for decades. The heroine is left deliberately open so a reader can fit into her. The effect is immediate: the look that lands on her lands on you.
The narration addresses you: “you are Jana” rather than “Jana was”. The lead has a name, an age, a job, but no trait that shuts you out. And her decisions are the points where you're asked.
Every story in the catalogue starts this way. You are Jana, twenty-nine, on the night train to Verona, and the compartment door opens. In most stories you play a woman, in some a man, in one a nonbinary character. Who you are there is fixed; what you do is not.
At Everglow you do not just read it. You are the protagonist, and every chapter ends with your move.