The balloon is inflated only to the point where it can still expand. Then a belly band, made of a non-stretchable material such as string or ribbon, is placed around it.

If you now ascend with the balloon in a balloon or gain altitude in some other way and the air pressure decreases, the balloon will continue to inflate. However, it cannot do so in the area of the belly band and changes its shape. This makes its expansion more visible than if it did not have a belly band.