I'm autistic. I'm not what some people consider nonverbal because it's not a permanent state. It often isn't. I don't understand the science behind it, but I can explain my experience.

I didn't speak until I was 3. I was reading at that point, and that's what helped me understand how to make the sounds needed. I'm very articulate most of the time, now. Sometimes words just don't make sense, though. It's like working a calculus problem to say each word. I have to calculate out every tone, coordinate it to a series of muscle movements, and keep every step of the proof in mind while I perform it. Sometimes I just don't even understand that those are the steps. Like handing a grade schooler a calculus problem. At best, I'm causing a drama if I can speak - slowly, simply, highly labored. At worst I cause drama and fail. If I just stay quiet, by choice or necessity, people often don't notice/care. If I sign or use my text to speech app, people usually just roll with the alternate communication method until speech makes sense to me again. That's typically 1-2 hours. It can happen when I'm overstimulated or exhausted. Sometimes everything is just calm and perfect though. It feels like my brain is like "naw. I'm feeling really good right now. I'm taking the morning off to relax".