Hola Lorenzo,

No sé hasta qué punto es fiable esta explicación (veré si puedo encontrar otra fuente mejor), pero creo que encaja dentro del contexto:

"Rapping" - this is when a horse is being trained to jump, two people hold the ends of the pole and has the horse jumps over it they raise it sharply so it raps the legs - this is supposed to teach the horse to pick his legs up higher. There are variations to this where the horse is wearing bandages containing pins etc so they are pushed into the legs from contact with the pole.
A characteristic of horses trained in this way is that they tend to pull their front legs so high that they are actually sticking out to the sides as it goes over the jump. ( but I think some do this naturally anyway )


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Note added at 21 mins (2008-11-24 13:31:09 GMT)
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Bueno, aquí figura en un artículo de The Times, creo que más fiable:

This issue has dogged showjumping for a long time. Hypersensitising is a formal term for what is known in Ireland and elsewhere in the equestrian world as “rapping”.

This refers to an illegal training practice where the horse’s shins are brought into contact with a pole or a stick as he’s jumping, to make him jump higher.

http://www.timesonline.co.uk/tol/sport/olympics/article45983...