Weighted pull-ups are the first step to getting stronger, but most lifters will quickly reach a plateau. Here's where speed work can come into play. Powerlifters have long used speed work to improve their bench, squat, and deadlift. The goal is to improve rate of force development, so instead of going heavy they'll use a lighter load and move it fast. Taking this concept and applying it to pull-ups, we get the band-resisted pull-up.
Speed Pull-Ups, Accommodating Resistance
Attach one end of a band to a heavy dumbbell on the floor directly beneath the pull-up bar. Affix the other end to a belt attached to your waist. The band should be taught at the bottom, but not overly tight.
Do pull-ups as normal, trying to do each rep explosively. Speed is key here.
Bands provide accommodating resistance, meaning there's less tension at the bottom and more tension at the top as the bands get pulled tighter. This forces you to pull explosively through each rep to avoid being pulled down by the bands as the tension increases.
How to Program It
Once a week, perform 6 sets of 3 reps in place of your normal pull-up workout. Do 2 sets each with a pronated, neutral, and supinated grip, and don't go anywhere near failure on any set. Add more band tension as needed, but err on the side of too light as opposed to too heavy.