1. JarPlug automatically extracts the file into the workspace. It puts it under a directory tree named $jarplug.work and further under a folder named test.jar.$work. Why? Because in Eclipse pretty much everything depends on resources having a real physical presence in the filesystem (see FAQ 285 in the Eclipse 3.0 FAQ Reference)
2. JarPlug opens the package explorer tree and selects the extracted file.
3. JarPlug automatically opens an editor for the extracted file.
For more information:
Recommend the tutorial