Lumbar extension is prevalent throughout the athletic and lifting populations. Fallouts are probably the most challenging anti-extension exercises out there. In the video, a single-leg variation is shown which adds an element of anti-rotation as well.
Set up in a slight forward lean.
Allow yourself to fall forwards without allowing your back to sag in the middle.
Keep your core firm and your arms straight.
Once you've gone out as far as your can comfortably, shift your weight onto your heels to return to the starting position.