Striking workers usually go unpaid and risk being replaced, so a slowdown is seen as a way to put pressure on management while avoiding these outcomes.
This refers to the "rule books" that govern workers' actions, usually either for safety or quality purposes.
This has the advantage of allowing workers and unions to claim that no malfeasance is being committed, since they are doing only what the management's rules actually require them to do.
In a "rule-book" slowdown, the bus driver may drive more slowly and conservatively and with a proper passenger load, while the pilot may refuse to fly in mildly inclement weather.
Slowdowns are related to but can be distinguished from work-to-rule, another action in which workers refuse to perform more than their required duties.