Overloading, is a technique that has been used for decades by power lifters and Olympic. Overloading does exactly what it implies, it places the body under more load than normally possible. This is done mainly by changing the range of motion of an exercise to be able to move that load. By overloading the body, we can strengthen the core tremendously as well as teach our body to lift heavier weights. It is great to add in every once and awhile, however, do not over do it, make sure the vast majority of your exercise should be full range of movement. Here is a great video example, give it a try sometime.
http://www.allthingsgym.com/overloading-partial-squats-slow-descent/