Subaru 2004 Impreza Owner's Manual page 311

Starting and operating
! To decrease the speed (by brake pedal)
1. Depress the brake pedal to release cruise control
temporarily.
2. When the speed decreases to the desired speed,
press the control lever downward in the "SET/COAST"
direction once. Now the desired speed is set and the
vehicle will keep running at that speed without de-
pressing the accelerator pedal.
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