If you watch the video at 1:05, the pipa players walk to their spot on stage, then place their left heel in a certain place and twist it to lock it in place (while you're watching the guy with the red scarf). At 2:50 you can see some of the rig used to support them on the player on the left's ankle, almost hidden by the flared Elvis pants.

It almost looks like a version of a "racer plate" ski binding that runs the length of the foot. Add some fiberglass or steel supports behind the calf and thigh and you've got an invisible chair. Those musicians look relatively light in weight, so the supports don't have to be bulky.

They can stand and sit upright very gracefully, but the person who started that illusion, a magician from Holland named Ramana has taken it beyond sitting, to appear in an almost reclining position. He has appeared in several airports in Europe reclining on a platform with no visible means of support, to advertise the seating space or extra legroom of some airline, I think KLM. There probably are some Youtube videos of him.

Or maybe it's Velcro. Or maybe it's magic?