rsync "-I" vs. "-c"

[Linda A. W.]

If I use the "-I" to ignore date and size as quick-check methods of 
determining
change, what method does it use to determine difference?  If it falls 
back to
checksumming the entire file, maybe the manpage might warn that this 
would be
as expensive as using the "-c" option...or not depending on what it uses for
determining difference at that point.

So exactly how does rsync compare files for differences when date & size are
used but checksumming is not?

Thanks!
-Linda