Called such because of the Windows key combination Alt-Tab to switch between running applications, alt-tabbing is the process of rapidly and covertly flicking these keys on the keyboard, usually in response to an authority figure approaching, to hide activity such as websurfing under a window containing actual work such as a spreadsheet or debugger. Alt-tabbing is a good way to mask procrastination while feigning greater productivity, provided you have fast enough reflexes. Alt-tabbing has generally superceded the Boss key provided in older singletasking games, which would pop up a fake "work" screen.
Some use the term to indicate rapid, covert application switching in other ways as well, such as clicking on the taskbar. It should also be noted that although it originated in Windows, the Alt-Tab combination now works in other operating systems, such as Sun's CDE, and KDE.1
This may be short, but it certainly falls under Node What You Know.
1 Thanks to Saige for pointing this out!