Here's what it does:
Just as you can simply right-click any Java project in the workspace and run all JUnit tests contained in it, you can now select any two or more projects and run all their tests as well. The only condition is that there is at least one project somewhere in the workspace that is a common dependency root of both (or all) selected projects, i.e. a project that depends, directly or indirectly, on both (or all of them). There is normally at least one such project, namely the one where you'd put your AllTests suite. Only of course with this new feature in Usus, you don't need an AllTests suite any more :-)
(A couple of remarks: If you select two projects where one of them depends on the other, directly or indirectly, there is no need for an additional project that acts as common dependency root. The depending project itself can play this role.
A Java project in the workspace 'depends' on another project when it has that project on its Java Build Path, as a required project. Among other things, this includes plug-in projects that are declared as required plug-ins when you do Eclipse plug-in development. Thus, if you do plug-in development, you can use this feature on any two plug-in projects as long as they are in the same dependency tree.)
There's a second interesting new thing you can do with JUnit test suites in Usus 0.6. You can also select a single Java project, and choose
Run As... All Tests Suite (JUnit). Usus will then collect all JUnit tests in that project and all its dependencies, i.e. walk the entire dependency tree
and create a test suite from all tests found.