Enable Pre-Roll Using ToolbarClick on the Pre-roll button in the Expanded Transport section of the toolbar. When Pre-Roll is enabled the Pre-roll button background becomes a brighter green.
Set Pre-Roll Amount Using Toolbar
Click on the numbers that are located next to the Pre-roll button and set the amount of pre-roll that you would like.
Pre-Roll Amount On Toolbar
Set Pre-Roll Amount Using Main Timebase Ruler
Drag the Pre-Roll flag on the Main Timebase Ruler to the desired location. In the following image the pre-roll flag is located at bar 2 which is one bar before the current playback position, thus setting a pre-roll of 1|0|000.
Pre-Roll Flag (small orange flag at bar 2)