This setting should be added at the same level as filter and is a boolean.
Depending on its value, the YUI configuration generated by the ez_platformui_yui_config twig function will instruct YUI to use a combo loader.

When true, the configuration should become something like:

{
  "combine":true,
  "root": "", // empty string
  "comboBase": "<base_uri_of_the_combo_loader>?", // for instance /combo?
  "modules": {
    "a-module": {
      "fullpath": "<path_to_module>,
      "requires: [<requirements>],
      "combine": true,
    },
    // ...
  }
}

When false, combine, root, comboBase and combine at the module level should be omitted. In a first step, the template modules won't have the combine flag.