Overloaded eccentrics – a fancy way of saying lowering heavy weights slowly – are a great way to recruit more motor units and muscle fibers, and to introduce connective tissues to extra stimulus. This makes the method very effective for increasing strength and triggering hypertrophy.
To overload the eccentric or negative portion of a pull-up, start with your chin already over the bar and your body loaded heavier than what you can pull up with). Then just lower yourself down slowly. Reset and repeat.
This method can be used...
If you're working on getting your first good pull-up, just use bodyweight. More advanced lifters will need to use supra-maximal weight i.e. strap on some plates or use a weight vest.
As a general rule, keep volume and frequency fairly low. These are meant to be done to high intensity (hence the low volume) and they're demanding on the structures of the elbow and shoulder joints (hence lower frequency). Try 2-3 sets of no more than 5 reps, once or twice per week.