Never heard of this before but, for me to get nostalgic I usually have to have some kind of external stimulation like an old video game or song or object like an antique toy or something or vintage magazine, TV show and etc. But, when it does hit and is strong the ache in my heart can be so intense that I want to cry because I want to go back to back then so bad. This can bring on great depression for me afterwards and then I try to find something to counter it. Usually not something healthy but, as long as it's something that calms me I go with it.