The Pop Up is a technique for getting up onto a bar or tree branch by using your forearms instead of your hands to get your torso over the bar. This bodyweight technique makes for a great Muscle Ups progression or alternative (the MovNat term for the calisthenics Muscle Up is Power Up). Practically speaking, the Pop Up is a more viable option than the Muscle Up when it comes to extremely wide surfaces like tree trunk-width branches.
For many people, the Pop Up is a very challenging movement in it's own right, so this short clip breaks this technique down into its three components, and introduces exercises for each.
These three components are pulling up to hook the forearms, pulling up with the forearms, and pulling the torso over the bar while externally rotating the shoulders, and pivoting on the forearms. Click here or on the image in this article to see the clip on YouTube.
Often, what's lacking in the ability to do the full Pop Up is strength. The exercises outlined in this short video build that strength in each segment of the movement, so if you know it's your pull up strength that's lacking, it might be the first two exercises you need to focus most on. If you have no problems doing the technique until the last part when you have to hoist yourself over the bar, then focusing on that last exercise on a very low bar with your feet touching the ground would be a good focal point.
The better your technique, the less you need to rely on that strength, so once you're able to do it, focus on how to do it with the least amount of energy expenditure.