This variation of pull-ups adds a bit more core involvement to the pull-up exercise. The form will remain the same as with standard pull-ups but with an extra movement.
You can perform the knee twist with one or both legs, but in either case, the knees will be raised up towards the abdomen and then crossed in either direction.
If using one leg at a time, the right leg will cross to the left, and the left leg will cross to the right. If using both legs simultaneously, the legs will alternate crossing to the right and left.
This knee twist can be added at the bottom and/or the top of the pull-up. It is important that you do not use this twisting motion to gain momentum for the pull-up. This variation needs control, both on the way up and on the way down. The addition of a knee twist will obviously create more stimulus for the muscles in the abdomen, but if done correctly, this extra movement will also utilize the latissimus dorsi.