We had a curved teepee of lexan or something like that. There were five spring-loaded knives that could be released from the backside. Unfortunately, I'm not a carpenter, so I can't really do a great job describing how it was built. The "knives" were wood and sat in a track behind the teepee. I can't remember what created the tension on the knife, which made it's natural postion to be sticking out of the front of the flat. The knives were preset by pulling the back into their tracks, and placing a pin in the track to lock it in place. A stagehand released them on the music cues for the knife throwing. Tommy just faked the throw and misdirection made it look real enough for a musical.