Arching the back provides a "bridge" which purports to optimizing bone alignment to allow the largest amount of weight to travel. The typical bridge leaves the persons scapulas locked in place underneath them with weight transfer then traveling along the shoulders and upper back on top end, and through the ass and feet on the bottom end. Taken to the extremes people do as this girl did, arching the back so that there are only 2 places the weight is traveling, the back/shoulders and the feet.
Feel free to read some powerlifting articles on improving 1 rep max for bench, where bridging is used the most. Bodybuilders tend to frown on it as it doesn't increase muscle strength and reduces range of motion. However, that is part of the appeal to a powerlifter. This move can reduce the overall range that the bar moves in height from the bottom of the movement where the stomach is touching bar to where the arms lock out overhead. If a regular bodybuilding bench is 15 inches and the bridge cuts half of that range out, there is less total travel for the bar vertically, which means that the muscles can lift a little more. As long as the person goes from the top of the movement to the bottom and then up it counts as a rep.
Now.. Whether it's legal in competition is another question. Depends on league, and this girl is Russian. I have no familiarity with their rules.
I wouldn't recommend using it as a technique. But, small lifters lifting big within their weight group tend to use it because it works. You have to think that when you're working within a weight class against other people of similar stature, every little bit of advantage in form or using your body's architecture and frame to reliably lift that week without injury, and without gaining more mass and ending up in a different weight class.