From ionite on discord:

is there some way to modify the independent throttle slider on engines?
enabling / disabling the toggle works, and setting the Independent Throttle field works, it just doesn't reflect anything in game

Need to investigate this and see if we can fix this behaviour.

When advanced tweakables are enabled, and independent throttle is enabled for an engine, there are two fields with the same name "Throttle" in ModuleEnginesFX. This causes issues when you try to get/set these fields, as the code assumes that names are unique, so it is ambiguous which field is referred to.

A possible fix is to use BaseField.name rather than BaseField.guiName, which I think is unique. This would break backwards compatibility of fields name though. (Maybe we should make the change, but provide some way to switch to using the old naming?)

We should also add control over the independent throttle settings to the Engine class, so that you can set it directly without needing to use part modules (which involves a lot more RPC calls)