Posner measures how well you can move your attention between different locations. On each trial, a small flash will appear on either the left or right side of the screen. Directly after this, a box will appear with a gap in the top or the bottom. If the gap is in the top of the box, press the TOP button, and if the gap is in the bottom of the box, press the BOTTOM button.
Because Posner asks you to switch your attention between both the different sides of the screen and the top and the bottom of the box, it not only measures your attention, but also how well you can move it. Because there is so much sensory data available to us in the world, our brains need to be able to focus, but also to move our focus rapidly to many different places.