A few months back I asked a similar question about this here:
http://www.pprune.org/pacific-genera...risations.html
When I was looking into the regs that were available on this topic I found that not much is publicly available, as YPJT has mentioned, however what I did read basically gave the impression that the "screener" can request you to do anything he or she deemed necessary for him or her to be satisfied that you are safe to pass the checkpoint. It seemed so loose worded that they could request that you lie down on the x-ray machine conveyor belt and scan you that way if they wished (I know that is a ridiculous interpretation of the rules but that is how it seems to read).
I spoke to a security person one day about various checking requirements and he mentioned to me that he worked the area that checks people accessing the tarmac for things such as baggage handling, maintenance, etc and he stated that in that area if he wished to look in someone's toolbox or similar that they had to ask the person in question to open up the bag and remove items as appropriate for the purpose of their screening and that they were not actually permitted to do this themselves. I did not think to ask at the time whether or not this also applied to passenger screening as we were discussing checked baggage and he just happen to mention that as well.
I do think that 95% of the time we do get a bit of leeway from the security screeners but we probably get our noses out of joint fairly easy over our being "screened" when something like the above mentioned situation happens, I know I didn't like it when one of them helped themselves to opening my bag, but I think I pass through security easier when I am in uniform than when I am paxing in normal clothes.