Everytime you kill a boss, you get 1 token added to your tokens section of your backpack. For every 10 tokens you get for that boss, you get 1 chance at the second chance chest. So, let's say you've gotten to Malistaire. When you kill him, you get 1 Malistaire Chest Token. After you kill him a 10th time, you can choose to throw in 10 Malistaire Chest Tokens rather than the 50 crowns.
Also, I wanted to throw this idea out there. It's a way to stimulate buying crowns. Every week, or every month, there should be a crowns sale! Double Crowns when you buy them, or redeem them that day! So, if you redeem a $10 gift card, or you buy $10 worth of crowns online, you will get 10,000 crowns, instead of the normal 5,000! I believe that more people will be willing to spend their real money for crowns if there is a benefit for doing it that day. Maybe you can just try it once and see how the sales go,
Professors, and my fellow players, tell me what you think!