How do you make a loop cut a straight line (not follow the contours of the mesh)? I was watching a tutorial video (Which you can find here) and at the 18:00 mark he creates a loop cut, then appears to press 'S' (scale) and mashes a few other keys that aren't displayed and the loop cut suddenly becomes completely horizontal and no longer follows the contours of the mesh.

What is happening here and how can I accomplish this, or something similar?
He presses SZ0, which scales the selected vertices to 0 on the Z axis, flattening them.

Another useful shortcut is E during edge slide, which aligns the new loop to one of the adjacent loops (press F to switch between loops).