>Psychtoolbox>PsychGLImageProcessing

rval = kPsychSkipSecondaryVsyncForFlip

This flag can be passed to the optional ‘specialFlags’ parameter of
Screen(‘OpenWindow’, …) or PsychImaging(‘OpenWindow’, …).

This flag will disable synchronization of OpenGL bufferswaps of a potential
secondary / slave window to vertical blank (“VBLANK” / “VSYNC”). Such windows
are used for the right-eye view in stereomode 10 (dual window stereo), special
dual-window display modes, and for display mirroring/cloning with the PsychImaging
task ‘MirrorDisplayTo2ndOutputHead’, which provides a copy of the visual stimulus
image presented to the subject also to a secondary window / display for the
experimenter.

Especially for this latter use case of display mirroring to an experimenter
control monitor, one usually wants to skip vsync and accept degraded image
quality / tearing, because tearing doesn’t matter much for a simple control
display, but disabling vsync prevents throttling of the main subject stimulus
display to the potentially lower video refresh rate of a cheaper/lower-end
experimenter display.