Selecting the button once activates the ALSD system. You're Auto LSD light along with the traction light will display. If you press and hold the button, the traction light and the VSC off lights will come on disabling the VSC, traction and ALSD...
Here's a pdf on the Tundra Star safety system. (More information is
here as well)
You can also visit the
Toyota i-Guide that will help you explain the features on the truck if you click on the "ignition, transmission, parking brake" section.
EDIT: You'll need to register for a TundraSolutions account to see the pdfs. I'm too lazy to save, upload to a host and repost (especially since I'm not home right now).