Playback and Recording : Audio Recording : Prime for Record Mode

Prime for Record Mode
When recording a large number of tracks or channels, or playing back a large number of tracks while recording, Pro Tools may take a little longer to begin recording. To avoid this delay, put Pro Tools in Prime for Record mode before beginning to record.
To enable Prime for Record mode and start recording:
1 Click Record in the Transport. The Record button flashes.
2 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 Record mode.
– or – 
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The Stop button lights and both the Play and Record buttons flash.
3 To begin recording instantaneously, click Play.
4 Click Stop to stop recording.
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When synchronizing to timecode, use Prime for Record mode to record or play back large numbers of tracks. This decreases the time it takes to lock to timecode.