Editing : Edit Modes and Tools : Using the Scrubber Tool : Shuttle Lock Mode

Shuttle Lock Mode
Shuttle Lock mode lets you use the numeric key-pad to shuttle up to two tracks forward or backwards at specific speeds: 5 is normal speed, numbers from 6 up to 9 provide increasingly faster fast-forward speeds, and numbers from 4 down to 1 provide progressively faster rewind speeds (4 is the slowest rewind Shuttle Lock speed, 1 is the fastest). If multiple tracks are selected, only the first two tracks are shuttled.
To play one or two tracks with the shuttle lock:
1 With Pro Tools HD, make sure the Operation preference for Numeric Keypad mode is not set to Shuttle (see Operation Preferences).
2 With the Selector tool, click in the track where you want playback to begin. To shuttle on two tracks, Shift-click in a second track.
3 Press the Start key (Windows) or Control (Mac) and a number on the numeric keypad: 0–9 (9 is fastest, 5 is normal speed, and 0 stops shuttling).
Once Shuttle Lock mode is initiated, Fast Forward and Rewind become highlighted in the Transport window.
4 Press additional keys to change the playback speed, or press Plus (+) or Minus (–) to switch the playback direction (plus for forward, minus for backward).
5 To stop playback, press Start+0 (Windows) or Control+0 (Mac).
To exit Shuttle Lock mode, do one of the following:
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– or – 
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