A double-layer background hanging on an axis of the photographic background is turned down, one is turned back, and the other is urgently needed. Thank you.

You're still not paying attention. The two background papers are hung in the same direction, and both sides can be put down!

The principle is simple. After passing through the reel, the lengths of the two background papers will be unequal, and the front (a) can be automatically put down and continue to rotate forward. A rotates half a circle around the background axis, and B replaces A, making B in front, so put it down!