Work to rule can be differentiated from a lazy workforce if it’s done well.
Typically, work to rule is used to highlight specific bad rules, regulations, or enforcement practices at a company.
Say a company expects employees to do non-rule “glue” work to keep the company functioning. But, randomly and capriciously the company punishes workers for doing this “non-rule” work. A union can then announce that they will only be sticking to the letter of the rules until either the rules are changed, or the arbitrary and capricious enforcement of the rule is changed.