Okay, this is NOT that exercise, but it's not unrelated.

Try "superman":

http://www.stumptuous.com/back-pain-3-exercises-for-low-back-pain (scroll down to reach it, and do note the thing about how some people find the compression isn't helpful for their backs, so YMMV)

To make it easier, move your arms back so they're alongside your body, pointing towards your toes.

(That's actually identical to a yoga pose called "locust".)

If that gets easy, try a back hyperextension -- I found this works really well at home if you lie with your hips on the swiss ball and your feet wedged under something:

http://www.stumptuous.com/good-mornings-and-back-hyperextensions

YMMV, but for me, unweighted back hyperextensions really helped keep my back in good shape after I'd had a run of lower back strains.