Turns formula auto-calculation on and off
You have a workbook with a billion sumif formulas, and it takes forever to recalculate every time you make a small change anywhere in the workbook.
Simply turn off the auto-calculation in the workbook, make your changes, and then hit the buttons again to turn it on again.
A message will show up in the Status Bar, at the bottom of your Excel window, stating whether the sheet was set to automatic or manual calculation, after you hit the keys to activate this toggler.
When auto-calc is turned off, Excel has built-in key commands to recalculate:
F9 = Re-calc everything Excel thinks needs re-calced.
Shift + F9 = Re-calc only the current worksheet.
Ctrl + Alt + F9 = Force re-calc everything and anything.