How do you stop an adjacent face from moving? I'm new to Blender, and really need some help. 

I'm following a tutorial, where you have a cube, and you use extrude region on one side of the cube so you end up with two cubes next to each other. You then select the top face of one of the cubes and push it downwards. 

In the tutorial the cube you don't touch remains as it was, so you have two cubes of different heights.

However, when I do this the top surface of the cube  I'm not moving, adjusts to my movement and slopes so the top half is like a triangle.

How do I stop the adjacent cube from changing it's shape when I adjust the height of the cube next to it?

I'm not sure if it would affect it or not, but proportional editing is disabled.

Thank you for any help you can provide!

Here is an photo of the part of the tutorial, it is from a magazine.



Step 8 says: You can move one face around independently of the adjoining face. Try pushing one of the adjacent faces downwards and notice that Blender fills the space between them with another surface.
For what it's worth : after using Blender for about a year, I have personally never encountered the behavior shown in that tutorial picture - that is to say, performing a "Sketchup-like" push and pull with an automatic fill. 

A third-party add-on allowing for something of that nature has recently been released, but this is definitely not default. This probably means that this tutorial has been badly edited/proofread.