You are close - the >>>
means it is going to fork a new execution of Maven. I couldn't find much detail, but you can look here http://books.sonatype.com/mvnref-book/reference/writing-plugins-sect-plugins-lifecycle.html. I think it's still synchronous from Maven's point of view, but just in a separate execution.
The plugin you're talking about spawns its own threads, so is inherently asynchronous, but that's not to do with Maven.
You can see the configuration of the appengine plugin here http://code.google.com/p/appengine-maven-plugin/source/browse/src/main/java/com/google/appengine/devappserver/DevAppServerAsyncStart.java - it's the @execute phase="package"
that means it forks.