Secret Professionals
The Modern Olympics upheld an "amateur ethos" by which professional sports players could not take part and Olympians were not allowed to receive any kind of payment.
From the very start of the Modern games this rule caused havoc. Athletes were stripped of their medals if private investigators detected a hint of professionalism. Jim Thorpe was stripped of his incredible decathlon and pentathlon golds in 1912 when it emerged that he had once played minor league baseball! (The medals were re-presented to his descendants in 1983)
Nations began dodgy gift systems as a way of indirectly paying their best athletes. The Soviet Union began "supportative" full-time training regimes for athletes, and by the late 1980s the International Olympics Committee took their head out of the sand and opened the tournament up to professionals.
s4sugar says
Clicking, making the sound with your tongue against the mouth, to encourage or get the horse's attention is not allowed during a dressage test. This is what was penalised at the 1932 Olympics.
Clicker training is entirely different and is a conditioning technique where the clicker sound, often made by a mechanical gismo, is associated with positive reward. It would not be used during a test but it's use outside the arena is not prohibited.