Contrary to popular belief, pushing the Refresh button (or Cmd-R) in Safari 4 does not really make it refresh. Well, it does, but there are at least two way of doing it, which send different HTTP headers:

Click Refresh: (Cmd-R)

Cache-Control: max-age=0

Shift-click Refresh: (not Cmd-Shift-R)

Yes, works with Safari 5.