I'm a brit, bough up on our version of traditional horse care.Strapping is tecnicall the whole grooming prosess and the 'thwacking' part is technically called whisping. It involves firmly but not shaply slapping a horse with a soft pad. This causes the horse no pain but does make them tense their muscale. Its like a primative version of thoses muscle toming machines we use on ourselves, you know? The ones that give you tiny shocks to tone your abs!
I don't know what vedeo on youtube you guys saw but try going to horseheo.co.uk, theres a really good vedeo on there that explains it all really well! :)