This mod is an extension of mod 6. This mod adds a spring adjustment, which allows you to adjust the cup seal spring tension. What this means is this, as you soften the spring tension on the valve, the valve opens farther with less force. There is a greater gas flow through the valve, which allows you to lower your operating pressure. The main point of this mod is to give you a fine adjustment with your velocity.
The Mad Marty's Spring Adjustment Mod is older, so while it may be available used and in a few cases new, it is not commonly available anymore.