Sniping and eBay proxy bidding work well together. While they are similar concepts, they are very different things, so it's important to understand the difference.
While we use sniping to place your bid at the last second, eBay uses their proxy bidding system to ensure that you only pay the minimum amount necessary.
First, you need to understand how eBay "proxy" (automatic) bidding works. When you place a bid, you enter the maximum amount you're willing to pay for the item. When someone else places a bid, eBay will add one bid increment to the price that is showing at the time of the bid. eBay will bid only as much as necessary to make sure that you remain the high bidder, or to meet the reserve price, up to your maximum amount.
Helpful Tip: For auctions, eBay always calculates the price by taking the second-highest bid and adding a bid increment.
For eBay's explanation, see their page for proxy bidding.
Here's an example:
Now add sniping into the picture. Sniping doesn't do anything fancy in addition to what you read above. BidSlammer just places the bid at the last second, exactly as you would if you placed the bid yourself. eBay's proxy bidding system kicks in to handle the rest--- that is, eBay will make sure you only pay the minimum amount necessary.