When you enter the command C-/, Emacs undoes some part of your recent changes to a buffer. When you enter C-/ again, it undoes another chunk of work.
I have read the Emacs manual entry on Undo but it is vague about exactly how it works. The manual says "Consecutive character insertion commands are usually grouped together into a single undo record" but it does not explain how it decides the number of character insertion commands that constitute a group. The number of characters it puts in a group seems random.
Can anyone explain the algorithm Emacs uses to group characters into undo records?