If I type this command through mongo shell db.test.explain().find(), I should get a json output about the explain. It doesn't work for some cases I am not sure why. After investigating I found that below is the explain.find function definition in javascript.

rs0:PRIMARY> db.test.explain().find
function () {
            var cursor = this._collection.find.apply(this._collection, arguments);
            return new DBExplainQuery(cursor, this._verbosity);
        }

compare with below command:

rs0:PRIMARY> db.test.find().explain
function (verbose) {
    var explainQuery = new DBExplainQuery(this, verbose);
    return explainQuery.finish();
}

it looks like the previous one miss finish() call. I don't know why this finish is not called in some cases on windows.