In the beginning we had no control over remembering or not. It was the debilitating flashbacks that initially drove our host to seek out therapy. The flashbacks - then and for several years after - were triggered by external events. The very first flashbacks happened when a child was born into the family and unbeknown to the host alters were afraid the child would be abused. (Our host up until that point had no knowledge or inkling of the abuse at all). Our host was really had no control over that process and was at the mercy of them for years.
I do it differently, but remember now we have had years of therapy and have developed a lot of internal skills. We still have intrusive flashbacks from time to time (recently have been having some from alters that we haven't had any communication with yet) but these days I am much better at actively communicating with the different ones and ask them to share stuff with me in a more controlled manner.

It has to be safe to remember though. We have had inside ones take over and shut things down because it isn't safe. If things are not safe to remember then it might not be able to happen.