To take a screenshot on an XO-4 with Android:
- press the power button and then X game key and hold them for a second, then release,
- a sound is played, a flash of white covers the screen, and an animation of the screen image shrinks away,
- a message "Saving screenshot..." appears in the top left of the screen,
- the image is available in a file browser or Gallery app,
It won't work if the X game key is pressed before the power button.
It won't work if the previous screenshot is still being saved.
(For further technical analysis, see ticket #12798. The X game key must be pressed within about a quarter of a second of the power button, and both must be held for about three quarters of a second.)
As a alternative there is a binary which can do the some thing via adb shell
adb disconnect && adb connect 192.168.XXX.XXX adb shell screencap -p /sdcard/screen.png adb pull /sdcard/screen.png gimp screen.png