In this poster, Saul Bass has used the Gestalt Theory principle of Figure/Ground. This principle states that as humans we separate foreground and background elements so we understand what it is that we are seeing. The viewer can manipulate/flip the figure/ground relationships, in this case, its the gun and the man’s outline, this is also known at “multistability”.
This logo by Dimitrije Mikovic is a good example of the Gestalt Theory Principle of Closure. Closure happens when there is a break in a design element, but our brains continue to perceive the image anyways. The viewer can see this upon looking at the break in the mountains, which actually helps to show the features and make the mountain look more rugged, and in the breaks, there is the shape of an airplane.