Tektronix TDS3012C User Manual page 144

Reference
Alternating trigger uses the current trigger settings to trigger on all
active channels; there is not a separate trigger setup for each channel.
Therefore, the trigger settings must be able to trigger on all active
signals in order to produce a stable triggered display. If one or more
of the source signals do not meet the trigger settings, the oscilloscope
either waits for that source channel to trigger (Normal trigger mode) or
autotriggers (Auto trigger mode).
Due to image persistence, all alternating trigger active channels may
appear to be synchronized. However, this does not mean that the
displayed signals are synchronized. Also, alternating trigger does not
use EXT, EXT/10, or Line signals as trigger sources.
Holdoff.
waveforms. After you push the Mode & Holdoff screen button, use the
general purpose knob to set the holdoff time as an absolute value or as
a percent of the record duration.
Holdoff begins when the oscilloscope recognizes a trigger event and
disables the trigger system until acquisition is complete. The trigger
system remains disabled during the holdoff time.
NOTE.
For best results, choose Normal trigger mode when using long
holdoff settings (10 ms or greater).
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You can use holdoff to help stabilize the display of complex
TDS3000C Series Oscilloscope User Manual