Playback and Recording : Playing Back Track Material : Playback Modes : Prime for Playback Mode

Prime for Playback Mode
When playing back a large number of tracks, Pro Tools may take a little longer to actually start playback. To avoid this delay, put Pro Tools in Prime for Playback mode before starting playback.
To enable Prime for Playback mode and start playback:
1 Do one of the following:
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Option-click (Mac) or Alt-click (Windows) Play in the Transport to put Pro Tools in Prime for Playback mode.
– or – 
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The Stop button lights and the Play button flashes.
2 To begin playback instantaneously, click Play.
3 Click Stop to stop playback.
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When synchronizing to timecode, use Prime for Playback mode to play back large numbers of tracks. This decreases the time it takes to lock to timecode.