Hands down is an English idiom meaning 'easily', 'without real competition', or 'certain'. It is especially used in the phrase 'winning hands down'. It is often hyphenated. Example usage: "He won hands down."; "The book is a hands-down best seller."; "You are hands down the best."
This phrase has been used since the 1860s to refer to an easy win. It originated in the sport of horse racing. Jockeys usually keep a tight rein while racing, to push the horse to run its fastest. When a jockey was well in the lead and about to cross the finish line, he would sometimes drop the reins and ride easily to victory -- winning with his hands down.
"There were good horses in those days, as he can well recall,
But Barker upon Elepoo, hands down, shot by them all."
-- 'Pips' Lyrics & Lays, 1867
Nowadays jockeys can't do this -- most racing commissions have a rule that demands that a jockey do all he can, during the entire race, to make sure the horse races its best:
Any jockey who, without adequate cause, shall rein in before crossing the finish line or who shall fail to use his/her utmost exertion to obtain a winning performance from the horse ridden by him/her may be suspended, ruled off, or may be subject to such other penalty as the stewards or commission may impose.
-- Ohio Administrative Code, Racing Commission, 3769-6-48
The general requirement is that every horse must be ridden in such a way that the Rider can be seen to have made a genuine attempt to obtain from his horse timely, real and substantial efforts to achieve the best possible placing. It is not necessary to use the whip to satisfy this requirement but a Rider must give his horse at least a 'hands and heels' ride.*
-- British Horseracing Authority, Instructions, H18 - 1. (b)
These days the term 'hands down' isn't particularly associated with horse racing, and can be used in any context. It has been used as a song title by Dashboard Confessional and as the name of a loud and violent board game (whoever hits the game board first wins) by Hasbro. I can only assume that these wonders of the modern era will be noded by someone more knowledgeable than I.
* The Debutante reports that while this may be the rule, it is not always followed in practice, and a rider will not be penalized if it is apparent that the horse was clearly going to win/lose.