The movement is the same as with a regular butterfly, only that the grip is narrow:

Sit on the butterfly machine, the feet have a good contact with the floor, the upper arms are parallel to the floor. Press your arms together till the handles are practically together (but aren't!). Go slowly back. The weights should stay all the time in the air.

Диаграмма показывает самые используемые мышцы в этом упражнении