Augmented reality systems commonly employ overt cueing to direct a user's attention and guide their actions. Spatial augmented reality based supraliminal annotations have been shown to improve user performance compared to LCD screens. This paper explores subliminal cues: annotations that exist below the threshold of consciousness. We investigate whether subliminal cueing is technically possible with standard data projectors, and if subliminal cues can further improve users' reaction time in procedural tasks. This paper describes a new technique for temporal subliminal cues in spatial augmented reality, and presents the results of three user studies evaluating the effectiveness of this technique in a button-pressing task. The results show that the presentation of annotations is indeed subliminal, in that the visual stimulus was not perceivable to users. We found a statistically significant improvement in task performance when using subliminal cues and supraliminal annotations, compared to supraliminal annotations alone, with mean trial times improving from 633.49ms to 604.64ms.