Page 269 - Buck Tilton - Outward Bound Ropes, Knots, and Hitches 2 ed.

Bring the turn now closest to your thumb

underneath the other two turns toward your
fingertips. By grasping the loop, you can now
remove the rope from your hand.

In addition to strength and security, the alpine butterfly
almost always unties easily, even after being heavily

loaded, something other loops fail to do. As with other
loops, this knot can be used to isolate— within the loop of
the knot—a worn or otherwise weakened point of a rope.

And it may be used in camping, boating, or any time an
object needs to be attached to a rope at a point other than

the end.
As with so many members of the knot world, this one can

be tied in several different ways. This way is relatively easy
to visualize.