What does “Force” do in Uncrustify?

Add means “add if not already present”, meaning that if something’s already there, leave it (and the formatting alone). Force means add if not present, and reformat if it is present:

// Original
if (cond)         {
    func();
}

// Add curly braces (already present, leaves formatting alone)
if (cond)         {
    func();
}

// Force curly braces
if (cond) {
    func();
}

Or another example:

// Original
if (cond)
    func();

// Add curly braces
if (cond) {
    func();
}

// Force curly braces (behaves just like add in this case)
if (cond) {
    func();
}