Knees to elbows is a core exercise that mainly works the abdominal muscles. Typically it’s done on a pull-up bar. It also involves forearm, chest and lat strength because you have to hang as you do it. It gets even more intense if you go for more reps and start kipping.
To do the exercise find a pull-up bar, or in my case a tree branch. You can even do it on gymnastics rings if you want to add some difficulty.
Start in a dead hang. Swing your legs behind you and bring your chest forward to start a kipping motion. As your legs swing up in front of you, bring your knees all the way up to touch your elbows.
On the downward motion go right back into another kip so you can keep a fluid motion going.
You can do the exercise without the kipping as well. I just decided to show it this way because it is more common. It also has been kind of adopted as a Crossfit exercise, so it’s a part of many WODs.