From the Atari Operating System User's Manual (1982) p. 215:
Attract mode is a mechanism that protects the television screen from having
patterns "burned into" the phosphors due to a fixed display being left on
the screen for extended periods of time. When the computer is left
unattended for more than 9 minutes, the color intensities are limited to 50
percent of maximum and the hues are continually varied every 8.3 seconds.
Pressing any keyboard data key will be sufficient to remove the attract mode
for 9 more minutes.
Laurent Delsarte contributes:
To launch the attract mode from BASIC, use a "POKE 77,128"
To disable the attract mode from BASIC, use a "POKE 77,0"