The memory shift modification is the most basic one to gain more performance/mining speed with most miners. The memshift versions do have modified memory timings to increase memory transfer rates at lower latencies.
What are memory timings and what do they do?
The memory modules on a graphics card work at a given clock speed measured in MegaHerz (MHz). Where each Herz is a cycle in which the memory has time to do calculations and work. The timing measures the chips delay doing calculations and work internally before passing the data forward. So even though a memory chip is working at high clock speeds, the work it does internally is limited due to the delays that are given by the memory timings. A chip working at 2000MHz with slow timings (large delays) can perform worse than a chip with 1500MHz and fast timings (short delays).
When the timing does not match a certain range for a given clock speed, the memory can run into trouble and generate memory errors and miscalculations. While memory errors do not harm the hardware, it can lead to faulty calculations of your mining software, thus in rejected shares.
The memshift roms are pretty straightforward as they use the tighter and faster timings of the lower clock speeds. For example, the timing from the 1500MHz clock speed is used at 1625, 1750 and 2000MHz. That results in higher transfer speeds of data from and to the GPU. And therefore to higher hash speeds of the mining software