An alpine butterfly is commonly used to tie in to the middle of a rope for glacier travel because it handles multi-directional loading well. For rock climbing, it can be used to isolate a core-short (when the rope sheath is damaged and you can see the core) in a rope.
Take the middle strand and pass it under, around, and through the other two strands.
Pull the strands apart.
The knot should look like this.
