How does the Capital One Purchase Eraser work?

The Capital One Purchase Eraser lets cardholders redeem miles for travel-related expenses made in the past 90 days, including airfare, hotels, and transportation. The cards with the Capital One Purchase Eraser include the Venture, VentureOne, Spark Miles Business Select and Spark Miles Business cards. To use the “Purchase Eraser,” log on to your Capital One account online or through the app. Click on your rewards miles, and then select the “Cover a Purchase” option. Your purchases that are available to be “erased” will show up, along with the number of miles required pay for each in full. You can also redeem miles to erase a portion of a purchase. Once you’ve selected the purchase(s) you want to erase, the miles will be immediately subtracted from your rewards balance. Your account will then be credited with the redemption(s) within 7 days.

The Capital One Venture and Spark Miles Business cards give 2 miles per dollar on all purchases. The Spark Miles Business Select card gives 1.5 miles per dollar, and the VentureOne card earns you 1.25 miles per dollar. Redeeming those miles for travel-related expenses gets you an exchange rate of 1 mile=1 cent. If you redeem your miles for a non-travel statement credit (not using the Purchase Eraser), they’re only worth half a cent each.

Most travel rewards credit cards are much more restrictive when it comes to redeeming miles. But Bank of America is one credit card company that allows cardholders to erase travel purchases like Capital One. After making a travel-related purchase, you can apply your Bank of America travel rewards to cover the cost of the purchase.

Capital One Purchase Eraser is pretty neat. So you make a travel related purchase, such as a flight, hotel stay etc. using your Capital One rewards credit card. Then you have 90 days to "erase" the purchase cost with your miles. The miles balance will update automatically and the credit will be reflected in your account balance within 5-7 business days.
They even have a cool commercial you should check out: https://www.youtube.com/watch?v=OLDhYlZRgoM