I'd love to see those contracts. They either have to put in a nice big juicy monetary penalty(If it's not your new employer will cover it) that each year at your pay review you can point to and say you obviously think me leaving is worth this much why am I paid 2-20% of it, or they chuck in a non-compete which are often unenforceable (unless they continue to pay you) or doesn't really matter to the hiring company as they can stick you with an unrelated project as they're not looking to capitalize on your skill as much as screw the other company. Really the only way I see it working is with deffered forms of payment which means more equity or performance bonuses. Maybe I'm just not imaginative enough.