What triggers saving SB's session data?

[Crenshaw Beildeurbeurghue]

It appears that merely clicking the Save icon, showing a downward arrow pointing into a box,  does not save a session.

Do I need to exit Chrome for an SB session to be saved in SB's session database? 

Also, does SB lose saved sessions when Chrome crashes? 

When Chrome crashed this afternoon and I went to SB to restore not the most recent session but a previous one created earlier today or yesterday, I discovered to my great dismay that I could find only the session that had been active at the moment that Chrome crashed. 

That is, none of the prior sessions that I thought that I had correctly saved appeared in SB's list of "Previous Sessions." 

BTW: I have been very frequently saving my tabs in Chrome using SB over the last several days and have kept Chrome open throughout.