Open blender. add a hair particle system to the cube object, and then go into particle mode and set the selection type to keys (second option of 3). Now select a hair particle key and press W->Rekey. The tool pops up, but the hair is deselected. To change the number of keys, you have to select the hair again, and then enter the number into the toolshelf. This also prevents the use of the increment arrows in the tool shelf, as each increment leaves the verts deselected again.