KIPPIS, ANDREW (17251795) , English nonconformist divine and biographer, son of Robert Kippis, a silk-hosier, was born at Nottingham on the 28th of March 1725

(I guess it's something cryptic regarding my 1000'th post)
Okay, I guess, we have to do it step by step. To repeat my Requirements/Use Case example:
- create a package with a use case, a requirement and a diagram. Draw a realize relation from the use case to the requirement.
- open the matrix
- click Source. You should see a project browser. Select the newly created package.
- Go to the Type drop down right of source and select Use Case as element type.
- Repeat the procedure with Target, but Requirements as element type. Now you have a matrix with Use Cases against Requiremets..
- select the type Realisation for relation.
- select Target->Source from direction.
You now should see a cross in the matrix. You can select that cross and from the context menu delete the relation. If you now switch to the diagram, you see that the relation has vanished. Back to the matrix you can re-create it by selecting the entry and creating it via the context menu.