As desert_rat implies there is a huge differnce in the ability to tolerate pain between the aroused and un-aroused states.
For example, I use needles a lot, very safely and aseptically. The insertion of a needle into either a very sensitive or less sensitive part of the body usually results in a bearable pricking sensation. Within seconds that slight pain combines with the erotic stimulation resulting from the doc handling perhaps a sensitive part, to produce an instant release of endorphins.
These hormonal derivatives have a two fold effect of mitigating pain which enables levels to be stepped up and also eroticising the patient. Thus the pain threshold can be elevated very considerably in stages.
To prove that many patients benefit in this way with absolute minimum pain I often use acupuncture needles, nearly as fine as a human hair. The insertion of these needles is barely perceptible on the pain scale but seeing the needles sticking into perhaps breast or nipple tissue can send the right subject into paroxisms of delight and pretty soon ordinary but fine hypo needles are going into the breast, nipple and even the labial area with accompanying and ever rising levels of pleasure.
Similar comments apply to clamping of nipples and other areas. When a clamp has been on a nipple for a while (10 mins max, please) the touch of a camel hair brush on the nipple can be a wow.
I have carried out many such procedures and my knowledge of this aspect of human physiology continually broadens. If anyone in the UK would like to test the above theories, in safety, confidence and within stated limits then please feel free to PM me, even to just discuss the subject.