In the negative pull-ups challenge you get to challenge gravity itself. Muscles grow stronger when contracting eccentrically (they lengthen) instead of when contracting concentrically (they shorten, what we call the usual "flexing of muscles"). Negative pull-ups require you to start from a full pull up position and slowly go backwards until you find yourself hanging, arms fully extended or what we call the dead hang position. The eccentric motion of the muscles (i.e. the lengthening of muscle fibers) generates more force at a lower metabolic rate (probably due to titin engagement in muscle lengthening). In addition, it engages, a lot of additional muscles such as core, abs, upper back and even lower back as the body's downward motion needs to be stabilized and controlled. The force exerted by the muscles recruits tendons in an active stretching capacity lengthening them as the muscles stretch and increasing their own strength and stability.
The dead hang position has additional strength benefits. In the first instance it increases grip strength which has become an important predictor of overall health and strength. It also helps develop fascial fitness which has important implications in the body's ability to tire from exercise and to translate muscle strength into power and explosive movement. At the end of the 30-day Negative Pull-Ups challenge you will feel in yourself a variety of changes in strength and endurance.