"Greasing the groove (GtG) is doing approximately half the number of repetitions that you are capable of, with 15 or more minutes of rest in between, troughout the day, every day, with an occasional day of". Pavel describing this. Here he mentions doing 2 reps when one could have done 4.
This sounds like high frequency training. According to Thibaudeau this works for about 4 weeks. After that the neural adaption tapers off. Sounds like a technique that is useful when learning to do pull-ups or deadlift.
However this article mentions using it for push-ups. Sounds like what is done in the military where you have to do 10 push-ups (when you can do 20) many times a day. A problem with this is that according to Henneman's size principle one must come close to failure on every set in order to recruit all motor units. The last 5 reps from failure makes sure all motor units are recruited. When you are only doing 10 push-ups where you could have done 20; muscle fibers that accounts for 25 % of your strength are not trained. That could mean that if you do huge amounts of push-ups you can do more push-ups but you are not getting stronger. However this mini study comparing GtG and another training method called Density for improving reps of push-ups and pull-ups simultaneously found that GtG worked very well and better than Density, at least for 3.5 weeks. From the data GtG seems to work for everyone but especially in the more technical exercise of pull-ups and especially for people who can not yet do many pull-ups:
This seems to indicate that increase in skills is partially why GtG works.
In this video Pavel explains that GtG makes your nerves become "superconductors" possibly trough synaptic facilitation or myelination. This article explains myelination as "a fatty white substance forms a sheath around the axons of nerve cells that allows the nerve impulse to move more quickly." Sounds good. However same article also mentions: "There’s little to no carry over in benefits to other exercises. So if you grease the groove with pull-ups, you’ll notice an improvement on the number of overall pull-ups you can do, but you won’t see much of an improvement in other exercises." This is not good and seems to contradict the theory that your nerves become "superconductors". If the nerves troughly became "superconductors" there would be a huge carry over since the nerves do not care if you do pull-ups or deadlifts.
Some exercises such as the pull-up has a huge skill component. I wonder if GtG has little to do with improving the efficiency of the neural signalpaths, but instead works by learning the skill faster trough frequent repetition. If so it makes GtG less great since the "strength" (in fact skill) you acquire does not have carryover.