There are many kinds of “kip machines”.
Good only at the earliest stages of learning, I like the emphasis on the arm action.
Kip Machine – Hang on the bar placing the feet in a tube or rope that is tied or hooked to the bar. Execute sets of straight arm kips standing up in the rope/tube to assist with the action. This drill can also be done while sitting on the rope/tube to increase the difficulty and more closely resemble the kip.
This sequence is taken from John Geddert’s article on kip and cast drills on bars: USA Gymnastics Online: Technique: Beginning Uneven Bars
video – Bars by John Geddert