A large body of research suggests that during learning motor skills, focusing on environmental effects of the movement (external focus) generally leads to better performance than focusing on one’s own body (internal focus). The functionality hypothesis states, in contrast, that the superiority of any attentional focus is task dependent. The present study aimed to test the predictions of the latter and searched for underlying mechanisms and task characteristics for one or the other focus being more functional. In Experiment 1, we examined whether the internal focus is superior in a difficult body-oriented balance task. In Experiment 2, we added visual feedback and investigated whether this would enhance the functionality of the external focus. In both experiments, the participants stood one-legged on a balance board and had to shift their centre of pressure (COP) to predefined target points. Per instruction, they were asked to interpret their attentional focus on the COP as either internal (the sole of the foot) or external (the platform). In Experiment 1, the external focus was induced through a mental image. The internal focus group performed significantly better, thereby supporting the functionality hypothesis. In Experiment 2, the COP was dynamically visualized on a screen. The internal focus superiority vanished. We suggest that the internal focus is more functional in motor-learning situations that provide more effect information through body-internal senses than through body-external senses. In these cases, the external focus hampers learning because it is associated with additional cognitive load.