This element of design is Negative Space. The hand isn't actually holding anything. It implies that there is a light bulb in the empty space. Negative space guides your imagination to finish the image.
WHITE SPACE: I love the simple style of this poster and how an image is made just using the hand and the screw. I think that it works well with the space and there is a focus point in the middle of the design.
This anti pepsi ad is a great example of semantic memory. You automatically know that this is the pepsi logo because of brand awareness. This shows that soft drinks such as this can make you fat so you must be careful what you put into your body.