Nathan,
When the bucket is flat on the ground, both the front cutting edge and the back of the bucket are touching the ground. When draging the bucket backwards, it will smooth out the ground you drive over. This is how I fill ruts, smooth gravel and just about everything.
If you tilt the front cutting edge up so it's slightly off the ground, but the back of the bucket is touching, it will dig into the ground to a small degree. I do this when I have a pile of dirt that's too small to dig out, but bigger then I want to smooth with the flat bottom of the bucket.
The more weight you have on the front bucket, the more effective it is. For this reason, I usually push the bucket all the way down, which results in my front tires rising off the ground. I can't steer, so I just go straight back until I either smooth out the area I'm working on, or I start to change directions and need to re-align myself.
On bigger construction tractors, they have a "float" feature that helps keep the bucket level when back draging. This is a normal, every day job for a backhoe operator, but for some reason, compact tractor owners seem to not do this and rely on the box blade.
Eddie