Make the pull-up harder and more effective at hitting the lats. But first...
Here's What NOT To Do
This is a pretty standard looking pull-up with the elbow tracking back and the lumbar in a big extension. Standard, but not optimal! The lumbar extension is what makes it more difficult to access the lats and easier to do the exercise.
Here's What To Do
Get your hips and knees flexed at 90 degrees. This position takes the extension out and places an emphasis on the lats. Track the elbows forward and down as opposed to straight down and back. This decreases anterior shearing force on the shoulder and brings the arm to the most optimal position for both injury prevention and muscle engagement.