Hold a foam roller between your thighs with your knees bent at 90 degrees. Now do some pull-ups. Get ready to be very surprised. Fan of kipping? You will be exposed. Lack torso and abdominal strength? You will be exposed.
Prepare to be humbled. Foam roller pull-ups can cut your numbers by 20-30%. If 15 pull-ups is your standard, don't be surprised if you only get 10-12 with this method.
Putting the knees at 90 degrees gives a co-activation of the lats, since part of the muscle originates at the hip. Add the foam roller for adduction and you get a double whammy of abdominal wall and adductor contraction that forces you to stay locked in and stiff.
This is also a convenient way to advance pull-ups without going on a search for a dip belt or weight vest to add load.