We Disneyphiles who go to Walt Disney World (and/or Disneyland) and elsewhere heard the same spiel over and over again - one about keeping our hands feet, legs, and arms inside the vehicle at all times to prevent affiliated injury. For Thorpe Park in Surrey, England, as they referred to the latter body part that should remain inside the vehicle, they birthed an additional item of note that should be added (or maybe not for most thrill ride enthusiasts) to the safety spiels - olfactory and hygienic etiquette to fellow riders.
That's right, the theme park chalked up another reason for riders to keep their arms inside the vehicle at all times - last month, they have to face crowds in the 29-Celsius (84-Fahrenheit) heat and they think that the basic staple of theme park ride safety would do the trick. Because the anticipation of queuing up for a really scary ride makes unsuspecting riders sweat due to the sympathetic nervous system (it's not just the heat), the odor would offset fellow riders. To help riders be considerate, they devised a scheme and implanted it in the park's rides - "Say No to BO!"
I don't think the rule is too plausible, at least to me because I usually hold on to the handlebars on almost every thrill ride at Disney and whatever park I go to whenever I can and I also follow the safety spiels and posted signage. As I queue up for the Big Thunder Mountain Railroad in the Magic Kingdom, I see other riders put up their arms like the people on TV or other media. That's a bit OK here in the States, but bend it like the runaway train ride on Stealth or Saw at Thorpe without heeding the warnings, and you'll be escorted off the ride. Do it repeatedly and you'll be expelled from the park without refund.
I think it's a bit fine for riders to put their arms up as they ride thrill rides, but I prefer mine, as well as many thrillseekers', just as the spiel says, inside the vehicle at all times, and the body odor issue is not the reason why, if you ask me.