System Configuration : Preferences : Operation Preferences : Transport Section

Transport Section
Timeline Insertion/Play Start Marker Follows Playback
When selected, the Timeline Insertion and the Play Start Marker both move to the point in the Timeline where playback stops. When deselected, the Timeline Insertion and Play Start Marker do not follow playback, but return to the location where playback began.
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Press Start+N (Windows) or Control+N (Mac) to toggle the Timeline Insertion/Play Start Marker Follows Playback preference on and off.
Edit Insertion Follows Scrub/Shuttle
When selected, the edit cursor automatically locates to the point where scrubbing stops.
Audio During Fast Forward/Rewind
When selected, audio is audible during fast forward or rewind.
Latch Forward/Rewind
When selected, fast forward and rewind latch and continue until you press Stop, Play, Return To Zero, or Go To End on the Transport. When disabled, the Fast Forward and Rewind only last as long as you hold the mouse after clicking either button on the Transport (or hold the corresponding switch on a Control Surface).
Play Start Marker Follows Timeline Selection
When enabled, the Play Start Marker snaps to the Timeline Selection In Point when you move the Timeline Selection, draw a new Timeline Selection, or adjust the Timeline Selection Start. When disabled, the Play Start Marker doesn’t move with the Timeline selection.
Reserve Voices for Preview in Context (Pro Tools|HD Systems Only)
When enabled, Pro Tools reserves the appropriate number of voices for preview in context (previewing audio files in DigiBase during session playback). The number of available voices is reduced by the channel width of the selected audition paths on the Output page of the I/O Setup window. For example, if the number of playback voices is set to 48 in the Playback Engine, and you have a stereo audition path selected in the I/O Setup, only 46 voices will be available for tracks. If you have a 5.1 audition path, only 42 voices will be available. When this option is disabled, you will not be able to preview in context if there are not enough available voices.
Custom Shuttle Lock Speed
Sets the highest fast-forward Shuttle Lock speed (key 9) for Shuttle Lock modes (Classic or Transport). The range for this setting is 50–800%.
Back/Forward Amount (Pro Tools HD and Pro Tools with Complete Production Toolkit Only)
Sets the default length of Back, Back and Play, Forward and Forward and Play. The timebase of the Back/Forward Amount settings follows the Main Time Scale by default, or you can deselect the Follow Main Time Scale option and select another timebase format: Bars|Beats, Min:Sec, Timecode, Feet+Frames, or Samples.
Numeric Keypad Mode
Numeric Keypad mode determines how the numeric keypad functions. You can always use the numeric keypad to select and enter values in the Event Edit Area, Edit Selection indicators, Main and Sub Counters, and Transport fields.
Classic
Selects a Shuttle Lock mode that emulates the way Pro Tools worked in versions lower than 5.0. With the Numeric Keypad mode set to Classic, you can play up to two tracks of audio in Shuttle Lock mode. Press the Start key (Windows) or Control (Mac), followed by 0–9 for different play speeds. Press Plus (+) or Minus (–) to reverse direction. Recall Memory Locations by typing the Memory Location number, followed by Period (.).
Transport
Selects a Shuttle Lock mode that lets you set a number of record and play functions, and also operate the Transport from the numeric keypad. With the Numeric Keypad mode set to Transport, you can play up to two tracks of audio in Shuttle Lock mode. Press the Start key (Windows) or Control (Mac), followed by 0–9 for different play speeds. Press Plus (+) or Minus (–) to reverse direction. Recall Memory Locations by typing Period (.), the Memory Location number, and Period (.) again.
Shuttle (Pro Tools HD and Pro Tools with Complete Production Toolkit Only)
Selects a type of shuttling different from that of Shuttle Lock mode. With the Numeric Keypad mode set to Shuttle, playback is triggered by pressing and holding the keys on the numeric keypad—playback stops once the keys are released. Various playback speeds are available in both forward and reverse. You can also recall Memory Locations by typing Period (.), the Memory Location number, and Period (.) again.
Use Separate Play and Stop Keys
When enabled, this option lets you start playback with the Enter key and stop playback with the 0 key on the numeric keypad. This is useful for quickly starting and stopping playback when auditioning loop transitions. The Use Separate Play and Stop Keys option is only available in Transport Numeric Keypad mode.
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When this option is enabled, it overrides using the Enter key to add Memory Location markers. Instead, press Period (.) and then Enter on the numeric keypad to add a Memory Location marker.