Zero-sum is basically a DKP system based on items, where the items have a predetermined cost before the raid starts (such as purple weapons = 30dkp, orange weapons = 50dkp, etc). When an item drops, the member who receives the item loses dkp equal to the items cost, and everyone else there gains their percentage of the items dkp value.
Pink Slayer Sword = 30dkp
Yookie gets the sword; Yookie receives -30dkp
10 other members were there when the sword dropped; they each receive 30dkp/10 members = 3dkp each.
Using Zero-sum with our system is very simple, create a new Raid template and select "zero-sum" for the dkp system field. Then create a new Raid using this template. To add items to the raid you fill in the items name, the items buyer, and the cost of the item. You then assign members who participated in getting the item. And thats it! All the dkp will be assigned and updated in the raid summary.
Zero-sum was added based on a feature request from one of our members; we are very open to suggestions and feature requests - if your guild would like to see any additions to our DKP system or any other aspect of the site do let us know =)