Complex sentence: The balloons are set up at sunset in the launch area, an open field, or a football stadium and are inflated as if they are going to take off, but instead of being allowed to ascend, they are held down by the ground crew.
Split into simpler sentences: The balloons are set up at sunset in the launch area, an open field, or a football stadium and are inflated as if they are going to take off. But instead of being allowed to ascend, they are held down by the ground crew.