Short stroking disks is a very, very old technique which was used to maximize the performance of the disk subsystems. The basic idea is that you only use the very outside edge of the disks so that the arm doesn't need to move very far on the platter to find the data that it needs to find. Additionally this keeps the platter from needing to rotate very many times to find all that data. The outer ring of the platter holds more data per rotation than the inner rings of the platter just do the simple fact that there is more surface area.
You can see this on the diagram of the platter below where the dark color ring is the part of the platter which isused and the light color part of the platter is not used.
Figure 1: Diagram of a disk platter where the dark ring is the only section of the disk being used as the disk is being short stroked.
This technique was very popular back in the 60s and 70s when disks were slow and people needed a way to make the disks faster. They would buy lots of extra spindles and instead of using the entire capacity of each disk they would instead only use the outer edge. Back when this was being done the disks where much, MUCH slower than disks are today. In almost all cases this technique simply doesn't need to be used any more. If your system is so hampered by storage performance that you are looking at short stroking the disks then you should really be looking at either higher end storage or flash based storage as a potential solution to your performance problem.