http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#JAR%20Index
Sounds interesting, and as it is based on an extra file in jars (IIUC) I don't see how it could cause any harm.
Did you do any measurements with and without this feature?
-Bertrand
http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#JAR%20Index