Hang from a bar with a slight back-and-forth swing.
On the forward swing, open your chest and shoulders, getting into a slightly Arch body position; this creates a stretch that will help give more power to your next move, thanks to the stretch-reflex.
Right after that, push your arms down and bend at the hips to bring your legs up.
On the way up, let your grip rotate on the bar and get your arms above the bar.
Extend your arms to go up and complete the exercise.
The first key component of this exercise is your pullup power. To increase it, practice weighted pullups and explosive pullups.
The other key component of this exercise is the technique. To develop it, practice it with an elastic band, which makes it easier and allows you to focus more on the movement.