Marcel Proust’s Concept of “Involuntary Memory”: What did he mean when he said that some of his memories felt involuntary?

Proust is often credited with coining the term “involuntary memory,” although he may have gotten the idea from the doctor who treated him when he was once hospitalized:

http://www.biusante.parisdescartes.fr/chn/docpdf/walusinskimemory.pdf

The term “involuntary memory” makes me think of two senses in which memories could feel involuntary: first, the posttraumatic flashback, in which the memory feels like your own past experience, but it is intrusive and frightening, and second, a memory that does not feel like your own memory—it feels like it belongs to another self, who had had the original experience, and is now remembering it for your benefit.

Keeping in mind Proust’s concept of multiple selves (see previous post), he may have mostly meant involuntary memory of the second kind.