OK, technically, this is not a sale on the Kindle Fire, but a sale on any electronics or toys at Amazon, provided you pay with a Mastercard. You'll save $10 on the purchase today (so, you charge $189.00 to your card, less any amount you currently have in gift certificates), then you'll get a $10 credit you can use on a future purchase of $50 or more (again, it must be paid using a Mastercard). All the details (such as they are), along with the promotion code you need to enter, can be found HERE. Note that you'll need to change your payment option to a Mastercard before attempting to enter the promo code, or it won't work, you must use the full checkout process (not one-click) and any gift card balance will be used first (see next).
So, are there any interesting loopholes? Actually, yes. If you have any gift card balance, it is used first - so long as your balance isn't enough to pay for the Kindle Fire, you can use your Mastercard to pay the remainder (presumably even if it is only a dollar or so) and you qualify for the deal. That gift card balance can be built up using cash (use your local money changing machine to get a gift card for Amazon with no fees), your debit card or even Visa (3x points if you use Amazon's Visa Card). Presumably you can do the same with the second purchase, which means you won't have to put much on your Mastercard (and can maximize your Visa points, if you want).
Evntually, you get a Kindle Fire with a net cost of $179 (assuming you use a free shipping choice), although that does presume you'll spend $50 later on at Amazon. It appears (but I can't confirm) that you'll be able to buy a gift card with the later credit (which would mean the entire $40 would have to be charged, though, as a gift card balance can't be used to buy another gift card), which would allow you to spend it on Kindle books (once you get the credit, you can test this before completing the order). It does say that it is a "one order" credit, so it likely won't work for Kindle books otherwise.
Note that this offer expires at Midnight tonight (on the west coast, US). If you are even a second after that, the promo code won't be accepted.