[amy214]

well you sit them down before they take the offer and level with them "If you come here I promise I'll treat you good and train you up for a bright future, but in exchange ideally I want you to stay for at least two years if you can help it, otherwise the cost of training doesn't work out"

It's not a legal agreement but a gentleman's agreement. They may still freely leave for whatever reason, especially if they were not treated well or treated like a slave (ie employer broke the agreement), but if all goes well it's some assurance, or at least a communication so everyone is on the same page. And usually being on the same stage and starting out with such trust sets the stage nicely for a good, trust-based safe working relationship