Bonus curbs are a cap on the amount of variable salary a worker may receive from the employer.
In principle bonuses work across any industry. Compensation for most sales related job involves a bonus. If I expect someone to earn a 50,000 package, do I pay them 50,000? No, I pay them 20,000 and a performance related bonus. If they perform well, they could very well earn more than 50,000. Without bonuses the only way for a firm to handle the ups and downs of the market would be to hire an fire.