— User Option: c-label-minimum-indentation

In ‘gnu’ style (see Built-in Styles), a minimum indentation is imposed on lines inside code blocks. This minimum indentation is controlled by this style variable. The default value is 1.

It's the function c-gnu-impose-minimum that enforces this minimum indentation. It must be present on c-special-indent-hook to work.

— User Option: c-special-indent-hook

This style variable is a standard hook variable that is called after every line is indented by CC Mode. It is called only if c-syntactic-indentation is non-nil (which it is by default (see Indentation Engine Basics)). You can put a function on this hook to do any special indentation or ad hoc line adjustments your style dictates, such as adding extra indentation to constructors or destructor declarations in a class definition, etc. Sometimes it is better to write a custom Line-up Function instead (see Custom Line-Up).

When the indentation engine calls this hook, the variable c-syntactic-context is bound to the current syntactic context (i.e. what you would get by typing C-c C-s on the source line. See Custom Braces.). Note that you should not change point or mark inside a c-special-indent-hook function, i.e. you'll probably want to wrap your function in a save-excursion¹.

Setting c-special-indent-hook in style definitions is handled slightly differently from other variables—A style can only add functions to this hook, not remove them. See Style Variables.