Here is a link to an interesting and informative talk about what “brainwashing” is. I found this talk by Dr. Zablocki to be concise and helpful in understanding what it is, when it is, and how we get influenced by it.
I was especially struck that I was not brainwashed into the cult, but rather the process was used to convince me to stay. I joined because it seemed good at the time. It took convincing to make me stay once I began to object to the practices. Those objections were a threat and had to be suppressed, and that’s where the pressure came from the leadership and the others in the group.