This is a great "eyeball" variety that is very easy to identify. As the engraver was working on the die, he first punched in the three digit "184" logotype. When he punched in the 6 as the fourth and final digit, it was turned 90 degrees in a clockwise direction. Upon discovering the error, the engraver simply turned the punch to the proper position, then pounded away, obliterating most (but not all) of the underlying digit.
Of the three major varieties for this year (Medium Date, Tall Date, and 6 over Horizontal 6), this is the most difficult to find. Mint State examples are very rare. As of July 2011, PCGS had certified 44 examples, 7 of which were Mint State. PCGS has not graded any 1846 6 over Horizontal 6 Half Dollars above MS-63.