no, it won't appear on the bid history (unless the bidding actually goes that high).
no, you won't get charged the full $51 (again, unless people bid up that high).
ebay's auto-bidding makes it so that the person with the highest max bid wins. using bcr's example: opening price is $1, I'm willing to pay 51$ so I put in my max bid of $51 since I'm the first bidder. Ebay only increments my bid to $1 to start. Then you come along and max-bid $75. The price would go up to the inext increment after my max bid of $51... so the item would now be at $52. if no one else bids, you win the item at $52, and no one would know what your max bid was.
capisce?
(lord... isn't there faq or something you can use?)