The “chicken wing.” DON’T DO IT! You risk serious injury by throwing one shoulder over the bar or rings followed by the other one. Getting that rep is not worth a lifetime of nagging pain. Injury to the rotator cuff remains for life. Even if you have surgery to repair a tear, it will never be the same.
Keep your shoulders squared when performing muscle ups on the rings or the bar. If you can’t perform the muscle up with good form, you need to go back to the basics. Work on your strict strength. Doing it properly takes months of hard work and repetition so be patient.
There are numerous muscle up tutorials on the web. This tutorial by Jerry Hill is a good beginner 101. Get your strict muscle up before you throw kipping ones. Your shoulders will thank me.