But how does it know when to take the snapshot? A clicked link can do all sorts of asynchronous things and Google is unaware when the new rendering is "finished".
Not necessarily. For instance, I would talk about a "database snapshot", meaning a "backup of a database at a particular moment in time".
Obviously the word "snapshot" originates from photography, but to me, the connotation is much more "capture an exact copy at a moment" than "make an image".