Overload/Underload Training is an old concept to break Speed barriers. The Overloading increases resistance and thus promotes using the big core muscles and the underload Training allows the Body to move faster than it can normally. It mostly works on neuronal aspects allowing the muscles to fire faster.
Soviet Trainers experiemented with that Kind of Training Long ago by having shot putters using heavier and lighter shots. Their studies found out that you should not go more than 20% over or under the Regulation implement because otherwise mechanics would be impaired. There is a lot of excellent Research by Coop deRenne regarding Baseball on that subject.
Note that this is an advanced Training method for breaking plateaus. I would only recommend doing this for older athletes with stable mechanics and good fitness. It is hard to make a rule but I would usually not do it before 14U or so. For younger kids working on mechanics makes much more sense.
but if you have good mechanics and some strength but batspeed is not progressing overload under load training can help.
so how is it done?
First you need a lighter bat that is about six ounces lighter than your game bat (you can use an old softball or a longer youth bat) and a bat that is about six inches heavier.
For making a heavier at use a wood bat, drill holes into it and glue fishing sinkers into it with epoxy glue. However make sure that you don’t change the balance point too much by grouping the weights around the balance point.
if you have those bats make sure that you are really warmed up. After that do some swings with the normal bat and then you are ready for the program. First do 5-6 swings with the heavy bat, then the light bat and then the regulation bat. Make sure every swing is max effort and take a little rest between the bats. Do 2-3 sets of this 3 times a week.
It is not unusual to gain 10 mph of batspeed in a few weeks the first time you do it. I went from high 60s in batspeed to 80 in about 2 months. However later the gains will be much lower because you both improve your mechanical efficiency and neuronal efficiency and those things improve rather quickly. For further improvement you might need to increase absolute strength or sometimes also mechanics. But for a quick boost this kind of training is very good especially for guys who have not trained for max effort. Again make sure you don’t do it before you have good mechanics.
Also make sure that you get enough rest. I made the mistake of doing it daily and while gains were fast first after some time neuronal fatigue set in and I even regressed a little. 3 times a week is enough.