This has the effect of pushing the clapper head towards you, to strike the back side of the casting. This also helps economize your effort. Backringing itself has several advantages. For one, the ringing motion can be performed at the end of the lifting motion, rather than making the lift, stopping it, then starting the bell moving in the opposite direction again. It's faster and easier on the muscles.
Some people find it easier to get really loud tones from the buckets by backringing.
Also, for those who feel a little uneasy about controlling the weight of the really big bells, the strongest part of the motion is towards the body, not away. Thus, if anything unfortunate should happen, you're all set up to catch the bell with your body, instead of the bell bouncing away from you and wreaking all kinds of havoc. Actually, it is not. The same circle is being observed, but the ring is happening at the top of the circle, not the bottom. For rapid passages, in fact, consider striking once at the bottom of the circle with a normal ring, then again at the top of the circle with a backring.