Mechanical drop sets (unlike regular ones where you just reduce the weight as you fatigue) allow you to switch to an easier modification of the movement, usually with the same weight. This helps you to increase metabolic stress and time under tension, both of which will stimulate hypertrophy.
If you use this during the main part of the workout, shoot for 3-4 mechanical drop sets. If you use this as a finisher, then one set is more than enough.
Do as many standard wide-grip pull-ups as you can. Use a weight vest or hang additional weight from a belt if you can do more than 15.
Next, go to volitional failure on eccentric (negative) pull-ups. Jump up to the top position, then lower yourself as slowly as possible.
Drop down under a suspension trainer or a bar and perform as many horizontal rows as you can (feet on the ground or elevated, depending on your level of fatigue). The suspension trainer is preferable as it allows your hands to rotate to maintain optimal shoulder position and to better engage the back muscles.