Yoko Ono "Voice Piece for Soprano"

Yoko Ono wrote "Voice Piece for Soprano" in the autumn of 1961. It reads:

"Scream.
       1. against the wind.
       2. against the wall.
       3. against the sky.

1961 autumn"

I found a video on Youtube of her performing the piece at MoMA in 2010. During the performance, Yoko Ono stands in the middle of a gallery at a microphone, and people gather in a large circle around her to watch. She goes through several different types of screaming/singing which sound sort of like vocal exercises, and she moves her body along with the noises.

Although it is interesting to watch, I cannot make the connection between screaming against something. As it is written, the piece sounds like it could be quite inspiring if performed literally. It would certainly have more context if done so, and I would love to see a performance of it. In the middle of a gallery, I think the piece loses any meaning. It seems like Yoko Ono is just screaming in front of bunch of museum-goers, and it is not very compelling to watch.

http://www.youtube.com/watch?v=7GMHl7bmlzw