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Yorker is a term used in cricket that describes a ball bowled (a delivery) which hits the cricket pitch around the batsman's feet. When a batsman assumes a normal stance this generally means that the cricket ball bounces on the cricket pitch on or near the batsman's popping crease. A batsman who advances down the pitch to strike the ball (typically to slower or spin bowlers) may by so advancing cause the ball to pitch (or land) at or around their feet and may thus cause themselves to be "yorked".
Origin of the term 
The term is thought to derive from the 18th and 19th century slang term "to pull Yorkshire" on a person meaning to trick or deceive them, although there is evidence to suggest that the Middle English word "Yuerke" (Meaning to trick or deceive) may have somehow been the source.
A batsman who has been "beaten" by a yorker is said to have been "yorked". "Beaten" in this context does not mean that the batsman is bowled or given out LBW but can include the batsmen missing the ball with the bat. A delivery which is intended to be a "yorker" but which does not "york" the batsman is known as an "attempted yorker".
As a batsman in his normal stance will raise his bat (backlift) as the bowler bowls which can make "yorker" difficult to play when it arrives at the batsman's feet. A batsman may only realise very late that the delivery is of "yorker" length and will "jam" their bat down to "dig out" the yorker.
Though the length of the delivery is key to "yorker", the classical reaction of a batsman to "yorker" reflects his surprise about the length rather than difficulty in playing that length. In fact, batsman find it difficult to play yorker because of misreading the length. The sharp downward deflection, due to gravitational pull, in the tail end of the yorker's trajectory cause batsmen difficult to judge the length. Whereas a flat delivery that lands near batsman's feet can be judged and played without much difficulty. Bowler has to take care of both length and trajectory to deliver a perfect yorker.
Use of the yorker 
A "yorker" is a difficult delivery to bowl as a mistimed delivery can either result in a full toss or half-volley which can easily be played by the batsman. Bowling yorkers is a tactic used most often by fast bowlers. A fast yorker is one of the most difficult types of delivery in cricket to play successfully, as the bat must be swung down right to the pitch to intercept the ball—if any gap remains between the bat and the pitch, the ball can squeeze through and potentially go on to hit the wicket, sending the batsman to the pavilion. The yorker might also not even hit the bat but hit the pads in front of the wicket, resulting in the batsman getting out LBW. When the batsman blocks such a ball, it is referred to as "dug out". A bowler who achieves swing when bowling yorkers can be even more dangerous, as the ball will deviate sideways as it travels towards the batsman, making it even harder to hit.
Yorkers can also be aimed directly at the batsman's feet, forcing the batsman to shift his feet while attempting to play the ball, or risk being hit. Inswinging yorkers have a reputation for being particularly hard to defend and difficult to score runs off. Such a delivery is colloquially known as a sandshoe crusher, toe crusher or nail breaker.
Despite the effectiveness of yorkers, they are notoriously difficult to bowl correctly and will usually be attempted only a handful of times during a sequence of several overs. Yorkers are best used to surprise a batsman who has become accustomed to hitting shorter-pitched balls and not with the bat speed necessary to defend against a yorker. As such, a yorker is frequently bowled quickly to give the batsman less time to react and position his bat.
The yorker is regarded as particularly effective against weak "tail-end" batsmen, who often lack the skill to defend even a non-swinging yorker and who are sometimes less susceptible to other bowling tactics. It is also particularly effective in the later stages of an innings in one-day cricket, because it is the most difficult of all deliveries to score off even if defended successfully. Runs will often only be scored off edges or straight down the ground.
Bowling a yorker 
A yorker is usually delivered very late in the action with the hand almost pointing directly vertical. The aim is both to get more pace and to deliver it later so as to deceive the batsman in flight. It is usually recommended to deliver the ball with some inswing but an away-swinging yorker aimed at the pads can be just as effective. Because yorkers are quite difficult to bowl they require substantial practise to achieve consistent success.