I always get questions about the "box" on the bull's back when I post buckin' pictures. I happened to take a great picture showing exactly how it works.
First we call the "box" a dummy. We use the dummy on young bulls that aren't rider ready. We begin bucking with the dummy as soon as the calves are weaned. Of course, they come in different sizes and weights. Mostly it simulates weight on the bull's back. But is also allows the flank to be pulled off. See below....
The red area is pointed at the flank. You can see the pin running thru the flank. When the dummy (blue arrow) is released by remote control, the weight of it coming off pulls the pin from the flank. It automatically releases the flank from the bull thus he will quit bucking.
You may ask "What's the big deal about getting the flank off?" Well, some bulls or heifers (cause we buck them too) buck so hard they could hurt themselves. If they come out of the chute and really buck, the dummy is released very quick. If you don't, they could break a leg or pull a muscle. So usually, the dummy is only on the animal for a few seconds. Just long enough for the stock contractor to see what the animal is capable of.
Now they are working on a contraption that would replace the actual bullrider. Stock contractors are increasingly having problems finding bullriders to get on their bulls for practice. We need to buck older bulls but when there's no one to get on, it makes for an impossible situation. But like I said, that's currently a work in progress.
So did that explanation help?