The description is correct, it shows number of requests that are currently being processes. So with a low load and very quick responses, it will most likely show 0.
One way to test is to use a servlet that will hold off a few seconds before responding to the client/balancer (e.g. clusterbench/clusterbench-ee7-web/src/main/java/org/jboss/test/clusterbench/web/load/AverageSystemLoadServlet.java at ma… accessed via http://localhost:6666/clusterbench/averagesystemload?milliseconds=20000&threads=4).
And really, -1 should not be there. It is a bug I have reported a while ago, see: https://issues.jboss.org/browse/MODCLUSTER-387 (it is probably not yet fixed).